Well apparently it has to do with its divisors.
A perfect number: its divisors add up to that number.
Deficient: sum of divisors is less than number.
Abundant: sum of divisors is more than number.
Divisors are like factors other than the number itself, apparently.
Divisors of 9 are: 1,3,3.
1+3+3 = 7.
So 9 is deficient because the sum of its divisors (7) is less than the number itself.
Check to make sure because I just learned by searching it up 2 minutes ago. I may be wrong.