Original: Interacting superstring theories are best thought of as configurations of an 11 dimensional supergravity theory called M-theory where one or more of the dimensions are curled up so that the line-extended charged black holes become long and light.
Simplified: Different superstring theories with different numbers of dimensions are best thought of as versions of an 11 dimensional supergravity theory called M-theory, where the extra dimensions are curled up, which means the strings become long and light.