An incentive for a tenant to sign a lease, "free rent" is a period of time, most commonly expressed in months, during which a tenant does not have to pay rent, but gets full access and use of their space.
Free rent can be "inside" or "outside" of a lease - Assess assumes that all free rent takes place inside of the lease term. For example, a three year (i.e. thirty-six month) lease that includes three months of free rent inside of the lease term is a lease that only calls for thirty-three months worth of lease payments. Conversely, if the same lease specified that the free rent were "outside" of the lease term, then the lease would still include thirty-six months worth of rent payments - but the space would actually be leased for thirty-nine months.
Free rent outside of a lease term can be represented in Assess by pushing back a leases's start date.