Let Slide
To let slide is to not call a bet from a player you think is probably bluffing. You decide that the pot is not worth the risk and you let the hand go.
The simple explanation
If an opponent bets and you suspect that he is bluffing, but you do not have enough of a hand to call, you let it slide. You fold and wait for a better spot.
A closer look
The phrase describes a choice that is often hard: you may be right that the opponent is bluffing, and still have a hand that cannot beat a bluff. Letting a bet slide is correct when your hand is too weak to win even against his bluffing hands, or when the price is wrong. Players who let too much slide can be bullied, so it helps to know when a bluff is likely and to keep some medium-strength hands to call with.
A practical example
A player holds J♥ 4♦ on a board with no pair or draw. The opponent makes a large river bet that looks like a bluff, but the player cannot beat any bluff and lets it slide.
Full explanation
To let slide is to not call a bet from a player you think is probably bluffing. You decide that the pot is not worth the risk and you let the hand go.
To let slide means not to call a bet that you think is a bluff.
You give up the pot instead of paying to find out.
It can be right when your hand is too weak to beat a bluff.
Letting too much slide makes you easy to bluff.