Please clarify the meaning of that?

For the best results you really should stick with a ratio around
9/21
5/10
3/7
And use pure inf, veh, or tank squads.
It is very easy for organizing your troops when you have 30+ full squads and you can split your units (if you use all 3 types) to fight people that have only inf, veh, or tanks.

No, it's how the ratios work that I don't understand. What does the numerator and denominator respectively stand for?

It means that you build all vehicles.
So if your building a squad of vehicles you build 5 armour vehicles (anti whatever you want) then you build 10 range vehicles(for each and every squad)
For tanks you build 3 armour tanks (anti whatever your building) and 7 range tanks.
Then for inf. you build 9 armour infantry (anti whatever your building) and 21 range infantry.
(Yes, if you do the math its 9-0-21.)
So for every squad you only put in vehicles or only inf. or only tanks.
Then you put the numbers previously stated of each type in that squad.

~Hero

sorry the ratio is armor to range less armor more range

Or you can always go for epic fail, aka pure damage squads.