The antenna clip is a small flexible part that snaps over the external antennas on the Terra to keep them from rotating (ie. from slumping over when pointed up).  The clip only works with two or more antennas.  Installation is simple: just orient the clip so the writing is face up when viewing the Terra from the front, and press the clip down over the antennas.  The friction of the clip will help keep the antennas in place.