A call gains value when the underlying rises. The buyer pays a premium and can lose at most that premium. The seller collects the premium and takes on the obligation to deliver shares if assigned (see assignment).
Calls are used for leveraged bullish bets, for income when sold against stock (covered-call), and as building blocks in vertical-spreads.
Example: a stock is $100. You buy a $105 call expiring in 30 days for $2.00 ($200 per contract). At expiration the stock is $112: the call is worth $7, a $500 profit. At $104 it expires worthless and you lose $200.
Related: put-option, strike-price, premium, covered-call, delta