A rent roll is a snapshot of a portfolio's income. A typical one shows, for each unit: the tenant's name, lease start and end dates, scheduled rent, the security deposit held, and any balance owed. Vacant units are listed too, so occupancy is visible at a glance.
Owners use it to spot leases that are about to end and units paying below market. Lenders and buyers ask for one when a property is refinanced or sold, because it shows the income the property actually produces.
A rent roll is only as good as the ledger behind it, so it should come from the same records you use to collect rent rather than a separate spreadsheet.