Runner Runner straight/flush draw = backdoor straight/flush draw = you have 3 cards necesary for your straight or flush, and you need to catch a good card on the turn AND on the river. Ex: you have T8
flop is 9 :spade:3 :diamond:A
You have a backdoor straight draw and a backdoor flush draw to the nuts. From what i can remember you make a backdoor flush draw about 5% of the time and a backdoor straight draw a little less, maybe 4%, depending on what type is it (if it's gapped, like 8TJ you have less, if it's 8TQ even less, etc...)
Stacking off= going all in
Crush a range = lol, see the english dictionary
Clean outs = outs that will certainly make you the best hand (nuts). For example if you have a straight draw on the turn you think you have 8 outs but if it's possible that another player has a flush draw you will have 6 clean outs (because of the 2 suited cards that complete that flush) and 2 partial outs (they may work as well, but someone could have made a flush).