A pair of squares is "mined" when stepping on yours while the enemy king stands on his loses to zugzwang; approach so that you step on second.
This position is one step before the trébuchet of the previous lesson. The squares c5 (for White) and e4 (for Black) are mined. In the trébuchet the side that has to move loses, so the king that steps onto its mined square first is the one that will be on move once the other king steps onto the partner square. Neither side wants to go first; both want to arrive second.