A "simple type" is a C++ type that can be constructed without constructor, destructed without destructor and copied without copy constructor or assignment operator. All basic types (like char, int or float) are simple. Pointers, references and arrays of simple types are simple. POD types ("plain old data types"), that are - simplified spoken - C++-types that already existed in C, are simple too.

Arrays of simple types can be copied very fast by memory manipulation routines, but the default implementation of functions like arrayCopyForward and arrayCopy are not optimized for simple types this way. But for classes derived from SimpleType, optimized variants of array manipulation functions are applied.

Note that simple types need not to be derived or specialized from SimpleType, but it could be convenient to do so.