Turnover is the whole cycle of ending one tenancy and starting the next: the move-out inspection and deposit reconciliation, cleaning and make-ready repairs, re-listing the unit, screening applicants, and signing a new lease. It is one of the largest controllable costs of operating a rental.
Its true cost is easy to underestimate, because the visible make-ready spend is only part of it. The rent lost while the unit sits empty often exceeds the repairs, and the re-leasing effort carries a cost of its own. Adding those three pieces — vacancy, make-ready, and re-leasing — gives the real number, which is usually why keeping a good tenant through a fair renewal beats chasing a small rent bump that triggers a move-out.
Estimating turnover cost per unit makes that trade-off concrete before you decide how hard to push a renewal, and a lower turnover rate compounds into higher net income over the years you hold a property.