myobject is an old-style functor. Here, since the () operator is overloaded, you can call 'myobject' like it was a function:
1 2 3
bool compare = myobject(3,5);
// here, 'compare' will be true, because 3<5
std::sort uses this as a mechanism where you can define how objects are to be ordered. It will take 'myobject' as a parameter and will "call it" (really, it will call the () operator) for elements in order to sort them.
std::sort (myvector.begin(), myvector.end(), myobject); // <- myobject is used as the functor here