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Call option is an option contract that gives the holder the right, but not the obligation, to buy predetermined quantity (usually 100) of underlying securities from the writer of the option at predetermined price (strike price) before maturity date of the option.
When market price of the underlying security is greater than the strike price determined in the option contract, holder will usually exploit the feature of the call option and exercise the option by purchasing the predetermined number of underlying securities from the writer, and sell them on the market for greater price.
Call option is the opposite of put option, which gives the holder the right, but not the obligation, to buy predetermined quantity of underlying securities from the writer at strike price specified in the contract.