What does a denominator tell us?

Answer 1

The denominator is how many equal pieces that one whole unit is divided into. The numerator tells how many of those equal pieces is measured. So in the fraction 3/5, the unit is divided into 5 parts and you have 3 pieces of the whole.

Answer 2

How many parts are needed to make one whole.

Or to put it another way, into how many [equal] parts the whole has been divided.