If you make money off a game that you’ve made with UE4 then you need to pay 5% royalty if you’ve made more than $3,000 in a quarter. That means during a 3 month period if you make more than $3,000 you pay 5% of your profit beyond that initial $3,000 to Epic.
If you’re just making games with your friends that you aren’t selling, then you don’t have to worry about it at all.