A unit rate is a relationship between an independant and dependant variable. For example, miles per gallon is a unit rate.
If your car goes 32 miles on 2 gallons, how far does it go on one gallon? To solve the unit rate, you say, "OK, I get 32 miles per 2 gallons, so simplify the fraction by dividing both sides by 2. I get 16 mpg."
This can also be written as 16 m / 1 g
I hope this helps! @-Happy7