C++ :: Move Constructor In Class Definition?

I am unable to understand how a move constructor works in this example of code. If someone could break down the process of what is taking place and explain to me on why to use a move constructor.

Code:
class MyString {
MyString(MyString&& MoveSource) {
if( MoveSource.Buffer != NULL ) {
Buffer = MoveSource.Buffer; // take ownership i.e. 'move'
MoveSource.Buffer = NULL; // set the move source to NULL i.e. free it
}
}
};

Example from "SamsTeachYourself: C++ in One Hour a Day"