Point multiplication using Montgomery ladder technique over Montgomery curves only require x coordinate, which in many situation leads to faster implementation as compared to point multiplication over standard curves. However, to compute point multiplication over Montgomery curves, a differential point addition operation is required https://www.hyperelliptic.org/EFD/g1p/auto-montgom-xz.html instead of typical point addition operation on standard curves.
What does this differential point addition means and how it can be computed?