clean outs are outs that are unlikely to help your opponents hand.

e.g. if you have 66 and the board is 789

then any T will give you a straight but it isnt really a clean out as anyone with a J has you crushed.

Similar with AQ on a board of TJA. any Q will give you 2 pair but it will give anyone with a K the straight so the Q is a very very dirty out.

edit: another less obvious, but probably more common example would be something like A9 on a flop of Q92

If the guy looks like he has TPTK then hitting the A would be horrible even though it looks like an out.