Plain English Meaning

It is one spot on the approach that does two jobs. It marks the start of the approach, and it is also the radio beacon and marker that confirms you are over the outer point of the final approach path.

Full Definition

A single navigation fix that serves two roles at once: it is a Locator Outer Marker (a low-power non-directional beacon co-located with the outer marker of an ILS approach) and it is also published as the Initial Approach Fix for that approach. As an LOM, it provides bearing information to the pilot via an ADF and a positive position indication when the passes over the outer marker. As an IAF, it is at which the published formally begins, often serving as the from which timed are conducted.