Dividend yield is the annual dividend per share divided by the current share price, expressed as a percentage. A 3% yield means $3 of dividends a year for every $100 invested. It's a quick way to compare the income different stocks pay.
Yield moves opposite to price: if a stock falls, its yield rises, and vice versa. A separate measure, yield on cost, compares dividends to what you originally paid rather than today's price.