If they had intended to march forth, they would certainly have made some preparation for it; but Allah did not like their going forth; so He made them lag behind and they were told “stay behind with those who stay behind.”
Meaning:-
Lag: to move or make progress so slowly that you are behind other people or things.
forth: forwards, onward or outward in place or space; forward: to come forth; go forth. onward in time, in order, or in a series