What exactly is a lower I've never built one it's my first time — 1 Answer

The "lower" refers to the lower receiver, which is the only part not included in this kit. For AR-type rifles, the lower receiver is the part of the gun with a serial number, therefore it is considered to be "the firearm" on its own for government purposes. You would have to purchase a stripped lower receiver through an FFL (such as a gun store), filling out the required paperwork and waiting the requisite number of days. Once you have that, this kit has all the parts that would go into that stripped lower receiver as well as the upper receiver and all the parts that would go into that as well. You can typically find stripped lower receivers from reliable companies like Anderson Manufacturing or Aero Precision for anywhere from $50 to $80.