A unit rate is a rate in which the second quantity, or denominator, in the comparison is 1 unit. You can find the unit rate by dividing the first quantity in a relationship by the second quantity.
Bonus:
The most common second quantity in rates, from personal experience, is the denominator in a fraction, like the 1 in 50/1. This is because the denominator is seen as the one specifying the unit. For example, in the rate "a car goes 50 miles per hour," the numerator would be the distance the car is traveling, which is 50, whilst the denominator would be the time the car takes to drive a certain distance - in this case, 1 hour. The first quantity is 50 miles and the second quantity is 1 hour. We can divide the first quantity by the second to get the fraction 50/1, which is a unit rate because the denominator (second quantity) has a value of 1. This also applies to values such as 90/3, where we would be trying to simplify the fraction to get a denominator value of 1.