What does it mean to have poor compatibility?

I’ve recently installed Citra and game I’ve always wanted to play was Inazuma Eleven 3: Lightning Bolt. When I first played the game, I got a constant 30 fps or lower occasionally. I constantly questioned myself as to why this happened. Can anyone explain to me how or why some games like these are poorly compatible with Citra Emulator?

Emulators compatibility list are subjective, each emulator community has a different criteria of what is playable/good/bad or not, and in the end, they are contributions, contributors give whatever rating they feel like, (although there is a “rating suggestion”) however, contributors usually give a rating based on how a game performs on their hardware/OS. different hardware means not only different performance but different or no issues, that can help you see why there are mixed ratings.