Plain English Meaning
It's an approach that lines you up with the runway using a radio beam for left-right guidance, but does not give you any up-down guidance. You manage the descent yourself using published altitudes.
Full Definition
A straight-in localizer approach: a non-precision instrument approach that uses only the localizer (lateral guidance) portion of an ILS, without glideslope information. It is flown to the runway aligned with the final approach course, using step-down altitudes rather than a continuous descent path.