Plain English Meaning

Stay at the altitude ATC just gave you. Don't start descending on the approach until your aircraft is actually tracking the localizer course inbound. Once you're on the localizer, you can begin the approach descent.

Full Definition

An ATC altitude instruction directing the pilot to hold the assigned altitude during radar vectors to the final approach course, and to continue holding that altitude until the aircraft is established on the localizer. Once established on the localizer, the pilot may then descend in accordance with the published approach procedure (typically to the next published altitude or glideslope intercept).