What's the plug-in(?) for taking items from inventory to the hotbar without opening the inventory?

Question

I've seen some YouTubers use what I think a plug-in, that either puts an item that they were just using straight from the inventory to the hotbar or an item right above the hotbar slot from the inventory (I'm not sure which one is it), without opening it. Does someone know what it is?  (also, wasn't sure what category to put it in)