typedef is defining a new type for use in your code, like a shorthand.
typedef typename _MyBase::value_type value_type;
value_type v;
//use v
typename here is letting the compiler know that value_type is a type and not a static member of _MyBase.
the :: is the scope of the type. It is kind of like “is in” so value_type “is in” _MyBase. or can also be thought of as contains.