What it means
A move-in concession is a financial incentive an apartment community offers to attract a new tenant to sign a lease. Common forms include: free rent for a number of weeks (typically 4-12 weeks on a 12-month lease), waived application fees ($50-100), waived admin or move-in fees ($150-300), reduced security deposit (sometimes waived entirely for qualified applicants), and free parking for a period.
Concessions are how apartment communities compete during slow rental periods or while new construction is leasing up. They're tied to occupancy — when a building has many empty units, concessions are aggressive; when the building fills, concessions retreat.
Critically, concessions usually don't appear on Zillow, Apartments.com, or other listing sites. They live in the leasing agent's email or the locator's spreadsheet. Renters who only search listing sites often pay full rent without realizing concessions were available.