The "lower" is the receiver plus some critical hardware like the trigger group and magazine retention.

A firearm's receiver is the foundation to which all other parts attach. According to the federal government, for most practical purposes the receiver is the firearm no matter how many parts you've changed or tweaks you've made. If you buy an AK-47 minus the receiver, what you'll get is a barrel, stock, gas piston, bolt, etc etc, a whole AK-47 in pieces, but nothing to actually put it all into: thus making it a box of replacement parts rather than a firearm. Big loophole.

This is also the reason why Tolly is looking to buy up spare lowers: if the AR-15 is banned as an "assault weapon", nobody will be making new receivers/lowers because the receiver is the firearm according to the feds. But "replacement" furniture and barrels and everything else you need to finish the rifle is a different matter.