Concrete expansion joint caulk is a special kind of sealant used to seal the grooves in between pads of concrete and render them water-tight. This protects the concrete from damage.
Concrete expansion joints are strategically placed grooves that prevent a piece of concrete from cracking. The most common kind of concrete expansion joints are the linear depressions seen in long blocks of sidewalk. Caulk, meanwhile, is a kind of sealant that is used to fill up seams and make them air- or water-tight. Concrete expansion joints are often sealed with rubbery caulk to give them an extra layer of protection against the elements. The caulk seals up the joint while still allowing it to serve its purpose of safely allowing the slab to expand or contract.