A rental concession is an adjustment a landlord makes to lease terms.
The adjustments are beneficial to tenants and may take various forms, depending on what a landlord is willing to offer.
Rental concessions will make a rental property seem more attractive to prospective tenants. A landlord may have a hard time finding a tenant or they may want to fill a vacancy quickly, so they'll advertise better terms or negotiate the rental contract to entice a tenant to sign the lease.
There isn't one single kind of rental concession and a landlord may make concessions at their discretion. They may choose to reduce rent, offer free amenities, spruce up the decor or whatever else they see fit.
As a prospective tenant, you can always propose concessions to the landlord and negotiate based on your wants and what the landlord deems acceptable. The worst thing a potential landlord could do is say "no," so don't be afraid to ask — just don't ask for too much.