Why does my bike randomly not work (when peddling wont go)

Awhile back my bike, when peddling would randomly become loose for lack of a better term. At the moment whenever stopping it would act as if the chain fell off but upon inspection the chain and gears are fine, whenever this would happen i would continue peddling till it "locked in" which i've been calling it. When this issue happend i looked at the gears and chain on the bike to find the problem but everything was moving fine, so i believe it has to do something with the back wheel and the rear gears.