A covered call is offering shares you own for sale at an agreed price on or before an agreed date, often 1 month or more in the future.
For example: if you own 1000 OSH (Oil Search) shares which you bought @ say $3.40 each ($3,400); you can offer to sell those shares at a strike price of say $3.50 each, in one months time. A buyer pays you a premium, for the offer [contract], not the shares, of say 90 cents per share ($900 per 1000), which you get straight away and get to keep. The buyer may exercise you at any time until the end of the month which means you must sell your 1000 shares @ the strike price of $3.50 ($3,500). If the buyer chooses not exercise you (usually when the shares remain sideways or go down), you keep the premium and the shares to write options over again next month.
Because shares can only go up, down or sideways, then its probable that you will not be exercised, most of the time; because you will only, normally, be exercised if the shares go up.