Container or carrier cover provides cover for additional costs that have to be made for continued carriage of the cargo if you are unable to complete the voyage yourself as a result of a cause covered by the hull policy. Both types of insurance actually provide the same cover, but for different types of cargo.
Container cover (as the name already says) is for containers and the carrier cover for dry, non-containerised cargo.
For tankers, for example, such insurance for continued carriage of cargo is not (yet) possible.

Example: Your ship is heavily damaged by a collision and cannot sail further. The recipient must, however, have the cargo available at a certain time, and additional costs have to be made - such as transhipment costs - to get the cargo to the place of destination on time.