I am not sure exactly. We would definitely need more context to answer your question. Namely, where did you hear this (what country and what type of controller) and when (was the aircraft establishing initial contact with ATC, departing a towered or a non-towered airport, approaching land after flying in oceanic airspace, etc).
Without the term "ADS-B" attached to the front, the instruction IDENT means that the pilot should activate the "IDENT" function of their transponder.
See this question: How does IDENT work? for details on why ATC would tell a pilot to do this and the mechanics of how this works.
Since ADS-B broadcasts are very similar to secondary radar replies, the IDENT feature works even when an aircraft is in an ADS-B-only environment (not within secondary radar coverage).
The term "ADS-B IDENT" may also refer to the flight identification which is continually broadcast by the ADS-B unit on the aircraft; this identification may be a registration number (like
G-ABCD) or airline identification and flight number (like
BAW123A). The controller may have been telling the pilot that the identification text being broadcast did not match the actual identification of the flight. This question provides some more information.