Plain English Meaning
An instrument approach that lines you up with the runway by using the signal coming out of the back side of an ILS antenna installed at the other end of the airport. It only gives you left-right guidance, not up-down, so you step down to published altitudes until you can see the runway.
Full Definition
A non-precision instrument approach that uses the back (reverse) signal of an ILS localizer antenna to provide lateral guidance to a runway opposite the one the localizer normally serves. It provides course guidance only — there is no glideslope — so the pilot descends using published step-down altitudes to a minimum descent altitude (MDA).