Normal concrete shinks as it cures, at least the stuff you and I can buy. It also changes size with temperature.
As concrete has nearly no tensile (pull apart) strength, it needs steel to hold in that direction. Steel is stretchy so it can't stop all cracking.
So keeping a slab wet and cool can eliminate most of the differential cooling cracks.
Rebar controls big cracks and helps pull the slab together as it shrinks. Mesh controls smaller cracking and can be designed to control larger cracking in place of rebar in the middle of the slab.
The welded wire mesh association has a really good design guide for sizing mesh for a given allowed crack size.
I know our 64 x 32 slab is going to shink a half inch to inch in the long direction so we can plan for that with control joints.
