In the second line, myvector.begin()+0 would be the 1st element, +1 would be the 2nd, +2 the 3rd, and so on. So to access the 4th element, it would be myvector.begin()+3.
In the last line, this is calling a different version of the same function (one that takes 2 parameters). The first parameter is the element to start at, while the second parameter is the element after the last one you want erased. So it will erase everything from the 1st element up to the 4th element, which is the first 3 elements.