In general, there should be a "liquidity premium" which means that less-liquid stocks should be cheaper. That's because to buy such a stock, you should demand a higher rate of return to compensate for the liquidity risk (the possibility that you won't be able to sell easily). Lower initial price = higher eventual rate of return. That's what's meant when Investopedia says the security would be cheaper (on average).
Is liquidity good? It depends.
Here's what illiquidity is. Imagine you own a rare piece of art. Say there are 10 people in the world who collect this type of art, and would appreciate what you own. That's an illiquid asset, because when you want to sell, maybe those 10 people aren't buying - maybe they don't want your particular piece, or they all happen to be short on funds. Or maybe worse, only one of them is buying, so they have all the negotiating leverage. You'll have to lower your price if you're really in a hurry to sell. Maybe if you lower your price enough, you can get one of the 10 buyers interested, even if none were initially.
An illiquid asset is bad for sellers. Illiquid means there aren't enough buyers for you to get a bidding war going at the time of your choosing. You'll potentially have to wait around for buyers to turn up, or for a stock, maybe you'd have to sell a little bit at a time as buyers want the shares.
Illiquid can be bad for buyers, too, if the buyer is for some reason in a hurry; maybe nobody is selling at any given time. But, usually buyers don't have to be in a hurry. An exception may be if you short sell something illiquid (brokers often won't let you do this, btw). In that case you could be a forced buyer and this could be very bad on an illiquid security. If there are only one or two sellers out there, they now have the negotiating leverage and they can ask whatever price they want.
Illiquidity is very bad when mixed with margin or short sales because of the potential for forced trades at inopportune times.
There are plenty of obscure penny stocks where there might be only one or two trades per day, or fewer. The spread is going to be high on these because the bids at a given time will just be lowball offers from buyers who aren't really all that interested, unless you want to give your stock away, in which case they'll take it. And the asks are going to be from sellers who want to get a decent price, but maybe there aren't really any buyers willing to pay, so the ask is just sitting there with no takers. The bids and asks may be limit orders that have been sitting open for 3 weeks and forgotten about.
Contrast with a liquid asset. For example, a popular-model used car in good condition would be a lot more liquid than a rare piece of art, though not nearly as liquid as most stocks. You can probably find several people that want to buy it living nearby, and you're not going to have to drop the price to get a buyer to show up. You might even get those buyers in a bidding war.
From illiquid penny stocks, there's a continuum all the way up to the most heavily-traded stocks such as those in the S&P500. With these at a given moment there will be thousands of buyers and sellers, so the spread is going to close down to nearly zero.
If you think about it, just statistically, if there are thousands of bids and thousands of asks, then the closest bid-ask pair is going to be close together. That's a narrow spread. While if there are 3 bids and 2 asks on some illiquid penny stock, they might be dollars away from each other, and the number of shares desired might not match up.
You can see how liquidity is good in some situations and not in others. An illiquid asset gives you more opportunity to get a good deal because there aren't a lot of other buyers and sellers around and there's some opportunity to "negotiate" within the wide spread. For some assets maybe you can literally negotiate by talking to the other party, though obviously not when trading stocks on an exchange. But an illiquid asset also means you might get a bad deal, especially if you need to sell quickly and the only buyers around are making lowball offers.
So the time to buy illiquid assets is when you can take your time on both buying and selling, and will have no reason for a forced trade on a particular timeline. This usually means no debt is involved, since creditors (including your margin broker) can force you to trade. It also means you don't need to spend the money anytime soon, since if you suddenly needed the money you'd have a forced trade on your hands. If you have the time, then you put a price out there that's very good for you, and you wait for someone to show up and give you that price - this is how you get a good deal.
One more note, another use of the term liquid is to refer to assets with low or zero volatility, such as money market funds. An asset with a lot of volatility around its intrinsic or true value is effectively illiquid even if there's high trade volume, in that any given point in time might not be a good time to sell, because the price isn't at the right level. Anyway, the general definition of a liquid investment is one that you'd be comfortable cashing out of at a moment's notice. In this sense, most stocks are not all that liquid, despite high trading volume. In different contexts people may use "liquid" in this sense or to mean a low bid-ask spread.
