Plain English Meaning
The runway is not as long as normal right now. ATC is telling you the available landing or takeoff length has been cut short, and you must repeat the words "Runway Shortened" back when you read the clearance so they know you got the message.
Full Definition
A specific phrase used by air traffic control to inform pilots that the usable length of a runway has been reduced from its published full length, typically due to construction, maintenance, or a temporary obstruction. When ATC issues a clearance involving a shortened runway, controllers append the phrase "Runway Shortened" to the runway designator, and pilots are required to read back the runway designator together with the phrase "Runway Shortened" to confirm they understand the reduced length is in effect.