TTL flash metering is a method of measuring light so you have better control over your exposures. It can be used with either a built-in or external flash. When a pre-flash is emitted, the light that returns and enters into the lens is measured by a built-in light meter to determine how much light is needed to achieve proper exposure. The flash output is then adjusted for the proper exposure of the scene.
Using TTL flash metering is beneficial because you do not have to calculate the flash output calculation yourself. Additionally, in some situations, there is no longer a need to carry a hand-held light meter when using TTL flash metering. This can save time when shooting scenes and reduce the margin of error.