Nothing is truly free. If you see an ad, talk to a person, or read an email (ahem) that offers you something for free, it's going to cost you something.
At the very least, you've already spent your time, energy, attention, and brainpower just considering whether or not you want the "free" thing.
If you decide to accept the free offer, you probably have to give up your contact information, some space in your home, or future time and energy spent with the thing.
When people say "free" they mean "zero money" (for now).
This is key to understanding the value of a dollar (or whatever currency you want to use). Money is a proxy for your time. You can trade your time for more money (work a job), or trade your money for more time (hire a person or tool to do something for you).
When it comes to cryptocurrency, network fees (gas) are a direct analogy of trading money for time. You spend tokens for network time (because the code you're running needs to run on every computer in the network).
The Koinos mana system is a different take on the same concept. Each token has its own time (mana). You spend the token's time (which recharges) and keep your money.
So, when I say "Koinos is free to use," I mean that it costs zero money to use smart contracts. You still have to spend your time (which doesn't recharge) and the token's time (which does).
P.S. Be wary of anything that claims to be free.