T-Stop stands for Transmission Stops and defines the actual amount of light that passes through a lens.
All lenses absorb a part of the light that passes through them. Depending on the lens, this absorbency can vary between 10% to 40% which means that the light that actually reaches the sensor can be 60% to 90% of the light that actually entered the lens. While It is a factor that can be ignored in still photography, It makes a huge difference in cinematography where we see a number of images in a rapid succession and even a small change in exposure will be noticed.
f/stop represents the amount of light that hits the sensor while T-Stop represents the amount of light that entered the lens. An f/stop number is always slightly less than the T-Stop number. For example, at f/stop 2.8, the T-Stop number will be 3.2 which means that a small portion of the light was absorbed by the lens.