Closed Cell Foams are such that the bubble within the foam is completely encased in the surrounding medium or material.
This makes them impervious to water or gas ingress. As opposed to open cell foam which is soft and pliable, closed cell is harden and more rigid in its structure. It also has a higher insulation rating (R-Value) than open cell foams, and certain types of closed foams are used in building insulation for this reason.
However, due to it requiring more raw materials in its construction, it can also be more expensive and heavier than open cell.
Closed cell foams have multiple uses, such as being used to prevent water or air from passing from one area to another, making them an ideal medium for plugging gaps and for other uses.