Someone tell me what this is. my teacher told me to write this down, but didn't explain it.

My guess is that when you get a world problem including fractions, the word "of" is going to be talking about the whole of the problem and the word "is" only talks about a part of it. For example, the fraction is 2 of 3. 3 is the whole since it correlates the the word "of". 2 is the part of the whole, because it uses "is." 2/3