This is one of those things that you don’t run into a lot, but it’s still nice to know what it is. A pour strip is usually 3 -5 ft wide area of concrete slab that is left open (or “un-poured”, if you will) in order to control shrinkage.
Why are we interested in controlling shrinkage? On large slabs, shrinkage can result in serious cracking. Therefore a contractor may pour two sides of a large slab first, leaving a pour strip in the middle. This allows the two slabs to shrink freely, reducing the slab’s tendency to crack. After a prescribed period of time (usually 30+ days) the slabs are hopefully done shrinking. After that, the pour strip is filled in with normal concrete to provide a continuous floor.