Impeding terrain is terrain which costs extra AP to enter. Press N when hovering over a space to see how impeding a space is. Slightly Impeding terrain costs +1 AP to enter, and Impeding terrain costs +2 AP to enter.
Note 2x2 champions only are affected by this if more than two squares of the destination are impeding. (This rule was added after the Nora Glades Map had a top font which was very difficult to contest with 2x2 champions.)