Re-entry is the process where an object comes back to Earth.
The heat (and comm blackouts) are due to the compression of air on the underside of the object. See, the object (missile, spaceship, rock, etc...) is moving so fast that air can't get out of the way fast enough so it's pushed down, which hits more air, eventually building up the pressure enough to super heat it.
The comm backout is primarily caused my ionization of the air as it's heated and compressed. Basically EM interference.