It just means their work is free use, as long as you give credit to them in some way. Like putting their username in a readme file that comes with the game, or in a credit roll at the end. And you don't need to pay the creator if it's free use, unless they explicitly state so (but then it's not free use, of course).
Pretty much, it's legal to use for no cost, that's why it's not locked behind a payment wall.
Not sure if I explained this well, and if I made a mistake, someone please correct me. ;)