Every option that gets bought also has to be sold by someone — and being on the selling (or "writing") side of that trade flips the entire risk shape around. A seller collects the premium upfront, immediately, as cash in hand. In exchange, they take on an obligation, not a right: if the buyer chooses to exercise, the seller must perform, whether or not it's favorable for them at that moment. A call seller who's assigned must deliver 100 shares at the strike price; a put seller who's assigned must buy 100 shares at the strike price — regardless of where the market price has actually moved.
Whether that obligation is manageable or dangerous depends entirely on whether the position is covered. A covered call means selling a call option on stock you already own — if assigned, you simply deliver shares you already hold, capping your upside at the strike price in exchange for the premium income, but with no additional, open-ended risk beyond what you'd already have owning the stock outright. A naked (or uncovered) call means selling a call without owning the underlying shares at all — if the stock rises sharply, the seller must buy shares at whatever the market price has become just to deliver them at the lower strike, and since a stock's price has no ceiling, neither does the potential loss.
Correct moves you up, wrong moves you down — reach 100 to master this lesson.