This is the potential income return on an investment, such as shares, bonds and funds. It's usually shown as an annual percentage. For example, if you buy shares in a company worth £1,000 and you receive a dividend of £20, the yield is 2%.
The yield changes with share price movements - a rising share price reduces the yield and vice versa. It also depends on the generosity of the company: the board can cut the dividend when times are tough, meaning a lower yield (boo!). But (and this is the good bit) companies are aiming to steadily grow dividends over time. So you buy shares in that company today, earning a £2 dividend or 4% yield. Then it doubles its dividends over three years. By then, you'd be earning 8% a year from your original investment. Nice.
With bonds, the yield is the bond's annual interest rate expressed as a percentage of its current market price.