For an AR-15, the upper receiver is the part of the firearm that houses the charging handle, bolt carrier group, and in some receivers also the forward assist and dust cover. Newer models of the upper receiver may also have the charging handle integrated into the receiver, rather than as a stand-alone part.
Most upper receivers also have a brass/shell deflector. However, some competition-level receivers do not.
Conversely, some upper receivers in an A1 or A2 configuration have fixed carry handle on top, while other receivers like the A3 or A4 do not.
The upper receiver also connects the barrel and handguard to the lower receiver, as well as connecting to the lower receiver to complete the firearm.