The tag is just a reminder about the fictional facts
It's not a mechanical instruction. It literally means that the weapon itself takes some kind of action — action in the sense of activity, not a game-mechanical spendable “Action” — by the fictional person holding the weapon to reload. It's describing a fictional fact about the gun.
Like, a crossbow doesn't reload itself. If you fire it, you have to reload it before you can fire it again. An assault rifle that has been emptied with autofire needs to have its clip removed and replaced with a fresh one before it can be fired again. This isn't a mechanical action, it's a fact of the fictional game world.
So, this puts restraints on what the character can do. When they autofire their assault rifle, it won't shoot anymore until they describe their character doing something to fix that. They might be able to just say they replace the clip, or they might not, or they might have to do something more complicated, dangerous, or slow to reload it. It all depends on what's happening around them at that moment.
For example, if the Savvyhead is pinned down by a brute sitting on top of him, but sees a pistol under the shelf beside him and manages to grab it, and shoot the brute? Great, that happens, and now the Savvyhead has a pistol and can shoot that other brute that just burst in to find out what the noise was.
But if the Savvyhead is in the same situation and it's a crossbow under the shelf instead, great, the brute is shot with a bolt through the neck (or whatever, there's probably a move triggered in there), but now another brute is bursting through the door and hey Savvyhead, you're holding an empty crossbow, what do you do now?