TTL stands for "Through The Lens". TTL literally means "Through the lens" when used with other terms. For example, TTL metering. It means that the metering is not a sensor mounted somewhere on the external part of the camera body, but is mounted inside the body receiving light information through the lens that is currently mounted on the camera.
TTL is important in functions where lens characteristics makes a difference. For example, if one lens is somehow darker cast (more tinted) than another, an external sensor would not know and will not be able to compensate. But a TTL sensor would be able to compensate.