The indefinite article is not used in Hungarian as much as it is In English. I see you study Russian; it's sort-of but not exactly like that. You can choose whether you want to use it or not. I'm not a native speaker so I can't say when you should, but it's not always used.
if you don't use it you are speaking more generally but if you do use it you are speaking specifically.
Játék can also mean "toy"
So if you say "ez egy jó játék" you would be more likely to be speaking specifically about a toy or game, describing it but maybe not playing it or using it.
But if you say "ez jó játék" you are more likely describing something you are doing or using at that moment. It can also mean it is fun to use, not just that it is good. It can also refer to any fun thing or event. Like how we might call a power tool a toy as well.