dragging a tab on top of another tab merges them into a "group" containing both of those tabs. it's like making folders on a phone's home screen, and it's exactly the same kind of fiddly because if you're not holding it dead center it assumes you're trying to rearrange them instead so it shoves them aside instead of creating the folder

dragging a tab all the way to the left and holding it there will pin it, a little pushpin icon will appear if you're holding it in the right spot