A dividend is a cash payment that a company makes to its shareholders. Many stocks pay dividends quarterly or monthly.
Apple pays a $0.25 quarterly dividend. If you own 100 shares, you get $25 quarterly.
Call Options: Slightly lose value on ex-dividend date. The stock drops by roughly the dividend amount.
Put Options: Slightly gain value on ex-dividend date.
If you sell a call and the stock has an upcoming dividend, the call is more likely to be assigned early (before the ex-dividend date). The buyer wants to be assigned to collect the dividend.
Related: Dividend Yield, Early Exercise, Covered Call