Buyback yield is the cousin of dividend yield. Instead of mailing cash to shareholders, the company spends the same dollars buying back its own shares. As the share count shrinks, each owner's slice of the business automatically grows.
A 4% buyback yield has roughly the same economic effect as a 4% dividend, with two differences. The gain is tax-deferred, because it shows up as a higher share price rather than taxable dividend income, and it depends entirely on management buying at a fair price. Buybacks at sky-high prices destroy value, while buybacks below the business's real worth build it.
Used alongside dividend yield, this gives the full picture of cash returned to shareholders.