A "unit rate" is a rate expressed in a quantity of one unit. A car traveling at a rate of 60 miles per hour, for example, is an expression of a unit rate. It describes the car's traveling rate over 1 hour of time.
Rates that use quantities other than one can be reduced to unit rates by creating a ratio. For example, if a person works 90 hours every three weeks, the rate of 90 hours in three weeks is equivalent to the rate of 30 hours in one week, written as "90/3 = 30/1." The unit rate is therefore 30 hours per week.