A coral dip is a short bath in a treatment solution that kills or dislodges pests and parasites on a new coral before it goes into your display.
Pests like flatworms, nudibranchs and "red bugs" hitchhike on frags and can spread through a tank, so dipping (and inspecting) every new coral is cheap insurance. Dip in tank-temperature water and never pour the dip into the display.