Plain English Meaning
Most ILS antennas put out a strong signal toward the runway pilots are approaching, but they also leak a usable signal out the back. A back course approach uses that rear signal to line up with the opposite end of the runway. There is no glide slope, and on a basic instrument the left/right needle works backwards unless the pilot sets it up correctly.
Full Definition
The signal radiated from the back (opposite) side of a localizer antenna, opposite the published front-course approach to the runway. When usable for navigation, it provides lateral guidance for an instrument approach to the runway end, but without glide slope information and with reversed needle sensing on conventional course indicators (a left needle indicates the is right of course, and vice versa) unless the pilot sets the published front-course inbound in the omni selector or uses an HSI configured to correct for it.