begin will return an iterator or a const_iterator depending on the const-qualification of the object it is called on.
cbegin will return a const_iterator unconditionally.
std::vector<int> vec;
const std::vector<int> const_vec;
vec.begin(); //iterator
vec.cbegin(); //const_iterator
const_vec.begin(); //const_iterator
const_vec.cbegin(); //const_iterator