I think the first misconception to clear up is that you are implying the price of a stock is set by a specific person. It is not. The price of a stock is equal to the value that someone most recently traded at. If Apple last traded at $100/share, then Apple shares are worth $100. If good news about Apple hits the market and people holding the shares ask for more money, and the most recent trade becomes $105, then that is now what Apple shares are worth. Remember that generally speaking, the company itself does not sell you its shares - instead, some other investor sells you shares they already own. When a company sells you shares, it is called a 'public offering'.
To get to your actual question, saying something is 'priced in' implies that the 'market' (that is, investors who are buying and selling shares in the company) has already considered the impacts of that something. For example, if you open up your newspaper and read an article about IBM inventing a new type of computer chip, you might want to invest in IBM. But, the rest of the market has also heard the news. So everyone else has already traded IBM assuming that this new chip would be made. That means when you buy, even if sales later go up because of the new chip, those sales were already considered by the person who chose the price to sell you the shares at.
One principle of the stock market (not agreed to by all) is called market efficiency. Generally, if there were perfect market efficiency, then every piece of public information about a company would be perfectly integrated into its stock price. In such a scenario, the only way to get real value when buying a company would be to have secret information of some sort. It would mean that everyone's collective best-guess about what will happen to the company has been "priced-in" to the most recent share trade.