It’s short for “永远的神(yong yuan de shen)” which means “ forever god”. It’s used to describe something very excellent ,very unique. For example, The milk tea that you’re drinking is very mellow, is the most tasty milk tea you’ve ever drank. You can say that the milk tea is YYDS!
You can see a lots of YYDSs in Chinese comments , because it’s one of the most popular Internet words among the youth of China.
Please kindly correct my mistakes.