Mastic is a special caulk that is water, chemical and UV-resistant. It is specially made for pool applications and is installed between the coping and the deck of your pool or spa.
It prevents weeds from growing in that space, but more importantly it prevents water from entering the space behind your pool shell, which can lead to damage to the shell or sagging of the deck.
Mastic installation usually takes 2-3 hours depending on the size of your pool and has a cure time of 24-48 hours, depending on current temperatures. Mastic comes in several different colors and designs.