Suppose a farm lets us pick 2 pounds of blueberries for 5 dollars. We can say:

blueberries (pounds) 
price (dollars) 
row 1 
2 
5 
row 2 
1 
$\frac52$ 
row 3 
$\frac25$ 
1 
 We get $\frac25$ pound of blueberries per dollar.
 The blueberries cost $\frac52$ dollars per pound.
The “cost per pound” and the “number of pounds per dollar” are the two unit rates for this situation.
A unit rate tells us how much of one quantity for 1 of the other quantity. Each of these numbers is useful in the right situation.
If we want to find out how much 8 pounds of blueberries will cost, it helps to know how much 1 pound of blueberries will cost.

blueberries (pounds) 
price (dollars) 
row 1 
1 
$\frac52$ 
row 2 
8 
$8 \boldcdot \frac52$ 
If we want to find out how many pounds we can buy for 10 dollars, it helps to know how many pounds we can buy for 1 dollar.

blueberries (pounds) 
price (dollars) 
row 1 
$\frac25$ 
1 
row 2 
$10 \boldcdot \frac25$ 
10 
Which unit rate is most useful depends on what question we want to answer, so be ready to find either one!