When buying a call option, the strike price of an option for a stock, for example, will be determined based on the current price of that stock. For example, if a share of a given stock (like Amazon (AMZN) ) is $1,748, any strike price (price of the call option) that is above that share price is considered to be "out of the money." Conversely, if the strike price is under the current share price of the stock, it's considered "in the money."
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).