You and your boyfriend are looking for a restaurant to eat dinner at. You pass by a restaurant that you've been to before. Your boyfriend hasn't been there, but you have. He asks if it's any good, and you answer:
Yeah, it's decent.
"Yeah" is a more casual way of saying "yes". Use it in spoken conversation when you don't need to sound too serious.
When you say that something is "decent", it usually means that it's good enough. In other words, it's good but not great.
A "decent" restaurant might be good enough to eat at if you're hungry and want to get some food quickly, but probably not good enough to go out to on a special occasion.
Here are some other examples of sentences that use "decent":
I'm a decent drummer, but mostly I play bass guitar.
It was fun. A decent number of people showed up, which was nice.
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