At face value, 'Market Capitalization' is a simple calculation:
Number of units in the market * market price = Market Capitalization
So the market capitalization of Apple is equal to the number of shares outstanding (860 Million) * the share price ($175) = 150 Billion. [I am ignoring complications regarding other non-common share instruments outstanding for Apple, for simplicity as this is a question about bitcoin].
But, what does it actually mean for Apple to have a market capitalization of $150B? Basically, it means that 'the market' (meaning, the average of all investors who trade apple shares, in particular the institutional investors like pension funds, who analyze these things and make the bulk of the trades) believes that the current value of all future Apple cashflows to shareholders (ie: dividends) is worth about $150B in today's dollars.
So if Apple announces a new desk with screens for legs and voice-command drawer openings, and investors believe it will make Apple a lot of money, then they will demand a higher price to sell their Apple shares. If the estimated value of Apple's future dividends increases by 30% in the eyes of 'the market', then Apple will now have a market capitalization of $200B. For this to happen, no one even needs to trade more than a single share. If one single share gets sold at $262, then that gets listed as the current market price for Apple shares. This means that a change in market capitalization denotes a change in the market's view of the value of all outstanding financial items. It does not necessarily denote an actual movement of cash.
Of course, it is possible for a cash movement to change the market capitalization of an item. For example: assume Apple runs out of new ideas, and they announce that they will continue producing current products, and once those stop selling, they will simply close shop. In the meantime, because they sit on a lot of cash that they use for R&D, Apple could pay out a massive dividend - say $10B. So if they did this, the market capitalization would decrease by $10B, because the company would be worth $10B less. Now this is critical: the price would likely then continue to drop, probably by over 50%, because people hold Apple shares on the assumption that Apple will continue to innovate. Most tech companies have the majority of their value in future anticipated earnings, far in excess of anything that they currently own or do.
Now if you look at something like a currency trade (say, betting that the USD will strengthen over the GBP), then the sum total of all currencies in the world effectively stay stagnant. These are ratios, not investments in themselves. If you buy GBP and the UK economy grows exactly the same as the US economy, then your GBP will be worth exactly as much USD as when you bought it (in general, for simplicity). But if you buy Apple, and Apple grows and IBM grows too, then your Apple shares are still worth more USD, even though IBM also grew.
What does this mean for bitcoin? Bitcoin is, remember, a 'currency'. Meaning it does not produce inherent value. It is only a medium of exchange. If bitcoin drops from $20k / BTC down to $10k BTC in the span of 1 month (hypothetically, of course), then the market capitalization will be cut in half.
So, the price drop that you might see in a given currency pair [including bitcoin, if you want to consider cryptocurrencies], is above all else simply an assessment by 'the market' that your chosen currency is not worth as much as it was yesterday. And this is why any type of foreign exchange trading, particularly unproven cryptocurrencies carries a high degree of risk, and is not suitable for long term investment.