Cool, I tried the discord link, but it stated invalid invite, this invite may be expired, or you don't have permission to join.
Err...Well anyway, If I am not mistaken, Royalty Free mean pay once to gain the rights to use for all type of commercial use, and no subsequent or additional payment required.
Not Royalty Free, means there will be ongoing or subsequent payment fee for every commercial game sold or depend on usage activity, sometimes, require separate or additional payment fee to get license rights to use like printing on shirt, banner, animation or videos or other sorts ... Every sale or profit earn, part of it have to be (certain percentage) royalties must be paid to creator or author. Or it can also be subscription fee depending on terms and license. I think that is what it meant.
Other than that, Is the asset only for Roblox development engine? Is it ok to use other engine like Unity, Unreal Engine, Godot or others?