a low is when you get have 5 cards 8 or lower.

e.g. if the board is KQ835, there is a low possible.

if the board is KQ833, there in no low.

and btw, straights do not count against you for the low, so if you have A2 in your hand and there is 345 on the board, you have the nuts low. however, if you get super unlucky, you would get quartered by someone who had like, A26 in his hand.