lhs and rhs stand for Left Hand Side and Right Hand Side in this context.
They are being used as inputs to operator over loader functions. The Left Hand side is being compared to the Right Hand.
Operator Overloader functions change the way operator symbols work.
Point2D operator+(Point2D lhs, Point2D rhs);
The above code will add (hence the +) the lhs to the rhs. Since Point2D is not a primitive numerical data type, C++ doesn't know to add two of them together. You are required to tell the compiler how the + operator should be executed for the Point2D datatype.