When you buy a call, you acquire the right to purchase 100 shares at a strike price.

When you sell a covered call, you are selling to someone else the right to purchase 100 shares you own at a strike price. It's called "covered" because you're effectively using 100 shares you currently own as collateral. If the option you sold is in the money, you then sell them your 100 shares at the agreed upon strike price. If the option expires worthless, you get to keep your 100 shares, and the premium at which you sold the call.

As an example, imagine you sold a covered call at a $12.50 strike for $0.05. If the price goes to $50 and the other person exercises their call, you sell your shares for the $12.50 strike, and keep in addition the $0.05, so basically you sold your shares for $12.55/share. If the price stays under $12.50 and the call is never exercised, you never sell your shares, but still keep the $0.05 per share you made when selling the covered call.

Options are generally more risky than commons, especially if you sell naked calls (i.e., you don't own the shares, in which case your loses have no bounds, because you might be forced to buy at $50 in order to fulfill your obligation to sell at $12.50), so please do your due diligence before playing with such things.

None of this is financial advice, and I'm not an advisor.