A monotone flop is a flop where all three cards are of the same suit. Examples are T76hhh, Q94ddd or J56ccc. Monotone boards generally slow the play down. You are less likely to see a player move all-in with an over pair on a monotone board unless one of their cards is of that suit such as AcAs on a 593ccc board. If you are faced with a move on a monotone board then you really do need to prick up those ears and pay attention, because it is not the type of board that players mess about on, even at the World Series of Poker.
On monotone boards there a fewer draws in your opponent's range. They are very unlikely to make a move holding an open ended straight draw or a flush draw that is not the nut flush draw. So when facing poker action on monotone boards you are very likely behind unless you are playing against an opponent with a history of bluffing on monotone boards.
Your mind will often play tricks with you on a monotone board. It tries to convince you that they are making a move with the lone ace of that particular suit. Whereas that can certainly be true, you have to play the percentages, and you will find your call will be right a lot less that it will be wrong.
So the general rule of thumb, when facing action on a monotone board, is to just let them take it down, because you are generally only beating a bluff.