TTL metering provides the values needed to properly set up your camera or flash by measuring the light in the framed scene. There's no need to worry about using manual mode, as the camera processes and converts all the necessary data.
TTL metering involves white balance as well as exposure.
Nowadays, the E-TTL subtype is popular, which differs from standard TTL with the addition of a pre-flash. The camera takes the necessary information from the illuminated scene and sets the device as needed, even before the photo is taken.
Some E-TTL flash units can communicate together thanks to the pre-flashes. This allows up to three flash units to work together simultaneously.