Concrete is very strong in compression but weak in tension — it cracks when it bends or shrinks. Rebar (reinforcing bar) is ribbed steel cast inside the concrete to carry those tension forces and hold cracks tight. In the US, bar size is given in eighths of an inch of diameter: #3 = 3/8”, #4 = 1/2”, #5 = 5/8”. Most rebar is Grade 60 (60,000 psi yield) under ASTM A615 and comes in 20 ft sticks (10 ft at home centers). A #4 bar weighs 0.668 lb per foot.
Placement matters as much as quantity. In a slab on grade, bars belong in the middle third of the thickness, held up on chairs or concrete dobies — rebar lying on the dirt does almost nothing. Concrete cover protects the steel from rust: ACI 318 calls for 3” where concrete is cast against earth (as in footings) and 1-1/2” for #5 and smaller bars exposed to weather. Where one stick isn’t long enough, bars are lapped — a common rule is 40 bar diameters, or 20” for #4 — and tied with wire.
Welded wire mesh (WWM) is a factory grid of thinner wires, most often 6x6 W1.4×W1.4 (formerly called 6x6-10/10) sold in 5’ × 10’ sheets or rolls. It goes down faster on thin patio and sidewalk slabs and helps control shrinkage cracks, but like rebar it only works if it is lifted into the slab during the pour.