Remember that fractions are used to count a part of something.
 

      of this square is red. 

Mixed numbers are used when you need to count whole things AND parts of things at the same time.

Check this out:

How much of these squares is red?

red square    red square    red square    

There are 3 whole squares and of another square...

We write it like this:

and read it like "three and one fourth."

It's really

But, we don't put the "+" in... This is why we say the "and."