I think this page explains it pretty well: https://www.topcoder.com/community/data-science/data-science-tutorials/algorithm-games/. Basically, the way you calculate Grundy Numbers (also called Nimbers) to a game state as follows. If it's a losing state (no moves can be made), assign it a a Grundy number of 0. Otherwise, consider all Grundy numbers of states that can be reached in a single move. Then, the lowest non-negative number that is not among that set is the Grundy number of the current state.