We know (almost all) objects expand when heated, then goes back when the heat is removed. Slabs are large flat concrete members that may be exposed to the sun. Examples of which are pavement, roads, basketball courts, sidewalks, etc. When concrete expands, you need rebar to stop it from breaking since concrete is not very good in tension. The concrete also undergoes stress when it shrinks (either from getting warm or from cooling).
Temperature bars are usually coined for rebars used on Slab on Grade (any slab that is not suspended). When designing suspended slabs, it requires rebars for flexural resistance. The computed amount of rebars takes temperature into account. Slabs on Grade do not have flexural rebars. But still needs rebar for temperature expansion. So “temperature bars” are added. At least in our building code its usually 10–12 mm dia rebars spaced at 300 – 450 mm depending on thickness.
The formula (rule odf thumb) is As = 20t, where As is the steel area and t is slab thickness. Which means a 4″ slab will require 12mm dia rebar spaced at 450mm or 10mm rebar spaced at 300mm.
Temperature bars are also used in one-way slabs. Since only the rebars on the short direction are computed, the rebar in the long direction are temperature bars. It uses the same formula above.