TTL means through the lens. Your flash will fire 2 times, once to measure how much light reaches the camera at a given standard flash power, and a second time when the picture is taken.
It uses the first flash fire to understand how much light is needed to expose the picture correct, and then boosts or reduces the amount of light in the second round.
It is a super convenient (automated) way to get the flash light right without any manual measurements with a light meter, or trial-and-error with a manual setup of your flash.
The big disadvantage of TTL is that your camera and your flash has to be able to talk to each other. The simple way to do this is to have the flash on camera, but if you want it to be off camera, you need to establish radio communication between the flash and your camera, which typically involves a flash trigger and a flash with radio receiver, so they can communicate with each other.
Another disadvantage is that the light your flash yields varies across a series of shots, as the camera may make different readings per shot due to slight variations in say available light. This can be a bit of a headache if you want to do consistent post processing of a large number of images, as you will have to adjust the exposure for each picture individually.
