Delayed pour strips are strips in a slab of concrete which are left as empty gaps when the concrete is first poured and then filled in later in a second pouring when the first pouring has dried. This is because some shrinkage is likely to occur as the poured concrete dries, and as this happens the gap you have left will expand a little. So when you fill it the delayed pour strip later with a second pouring the concrete will already have settled into its final position and you are less likely to have cracks forming. It's rather like the principle of an expansion joint.

See this file, page 52, where there is an illustration that makes it fairly clear:
"Delayed pour strip concreting
Fill the strips after sufficient drying shrinkage has taken place in the slabs [...]"
http://www.lensim.co.rs/files/File/Penetron Precast Tank Man...