It’s something I’m not too familiar with. But, as far as I can tell, there are no free characters. All characters (dubbed Axie) must be purchased with real money, with the cheapest costing roughly US $26.
I imagine it to be a PVP game in which your three-person team must battle and win in order to “earn” money. (I haven’t tried it because I become quickly addicted/distracted by new games.) It appears to be a form of mobile gaming gambling, which is a little suspect.