T - Tread
Tread - Your tyres have grooves moulded into them to help them grip the road, and the tread is the area which actually touches the road. Tread depth is the depth of the grooves.
Legally, the tread must have a depth of at least 1.6mm across the central three-quarters of the breadth of the tyre and around the entire circumference.
If the tread is allowed to wear down further than this, the tyres will not be able to grip effectively, and you could suffer a skid leading to an accident, especially in the wet.