This is a unit rate. It is saying how much of one thing to how many of a single thing, aka how much of one thing to a single thing, known as a unit.
This can be seen in prices. For example, if you can get 10 boxes of pasta for 10 dollars, then the unit rate is 1 dollar per 1 box. (btw, thats a good deal. I wish I had that :P)
Hope this helps!