An option remains valuable only if the stock price closes the option’s expiration period “in the money.” That means either above or below the strike price. (For call options, it’s above the strike; for put options, it’s below the strike.) You’ll want to buy an option with a strike price that reflects where you predict the stock will be during the option’s lifetime.
When buying or selling options, the investor or trader has the right to exercise that option at any point up until the expiration date - so simply buying or selling an option doesn't mean you actually have to exercise it at the buy/sell point. Because of this system, options are considered derivative securities - which means their price is derived from something else (in this case, from the value of assets like the market, securities or other underlying instruments). For this reason, options are often considered less risky than stocks (if used correctly).