[Kelvin]

Consider that when you load any kind of floor it would tend to deflect downward. In so doing, the floor -- and in this case -- the concrete slab would develop compression at the top surface, but tension in the bottom face. This condition is especially dangerous for concrete floors, because it is much weaker in tension and will lead to cracking.

To prevent a dangerous condition, we lay steel bars near the bottom face so that if cracks do develop (from excessive loading), a tensile-capable material is present to bridge the cracks and carry tensile forces beyond the cracks.

Alternatively - a prestressed or post-tensioned concrete system not only adds steel to the concrete where it is needed most, but it is pulled very taught so as to induce a compressive stress field into the concrete. If the magnitude of the compressive stress applied into concrete is larger than what would be developed in tension (as when our floor gets loaded), then the concrete never cracks and never becomes dangerous.

The difference between Pre-stress and Post-tension is merely when the steel is tensioned w.r.t. when the concrete is poured (i.e. before or after).

If the degree of post-tensioning is large, then it is also possible to make slabs thinner. It is this fact that helps make economical in highrises, where transporting concrete upward is done at a premium.

Tempered glass works on the same principle.