Proration means charging a partial month’s rent when a tenant moves in or out mid-cycle, so they pay only for the days they occupy the unit. If someone moves in on the 20th, you prorate that first month rather than charging a full month’s rent.
The common method is to divide the monthly rent by the number of days in the month (some landlords use a flat 30), then multiply by the days occupied. Exactly which convention applies can be set by your lease or by local practice, so state the method in the lease to head off disputes.