1. What is TTL Tax, Title, and License Time to live
Transistor-transistor logic
Through the lens metering
Time to love

2. Through The Lens metering / flash
A camera feature that permits light metering through the taking lens itself, rather then via a separate window- wiki
In the case of flash photography- a system that senses exposure requirements by pre-flashing the subject and adjusting shutter speed or aperture to arrive at the proper exposure.

3. How does it work. Without flash
By pressing the shutter button to halfway the camera examines the ambient light levels, determines the proper shutter speed, exposure and in full auto, ISO, this is typical auto exposure operation. ( note it also sets the focus if in auto focus )

4. How does it work- with flash
With flash it fires a short pre-flash and once again assesses how much the levels in those areas have changed. After that the camera calculates how much light (flash duration) it needs to arrive at the proper exposure it sends that back to the strobe and sets the proper flash duration
Please note : that a strobe output never changes just the time duration of the flash

5. How does it work ?
Kind of amazing considering that during this period the camera
1) auto focuses,
2) determines the exposure setting,
3) calculates the additional light needed and
4) sets the duration of flash for the strobe to obtain proper exposure.