The phrase "for good" can also mean "in good/positive ways instead of bad/negative ways": Suppose there is a very rich man named Paul. Paul spends his money on getting drunk and hiring prostitutes. Then he has an accident on his yacht (i.e,. large, expensive boat) and almost drowns. We might say, "After his near-death experience, Paul began using his money for good." Maybe Paul begins to spend his money to help poor people or to find a cure for cancer. There is a well-known musical play called "Wicked." It is about the two main witches from "The Wizard of Oz" (Elphaba, the green witch; and Glinda, the white witch) when they were young and went to school together. One of the most popular songs from the play is called "For Good." The song is about the two friends and how they have changed because they knew one another. They say that they have changed "for good." The words mean TWO things at the same time: "I have been changed in good/positive ways." ("You made me a better person.") "I have been changed permanently." ("I will never be the same person I was before.") You can hear the song with lyrics here: https://www.youtube.com/watch?v=uzrGFQysfYU (It may not be easy for a non-native speaker to understand all of the words; but everyone can understand the main words: "I believe I have been changed for the better; and because I knew you, I have been changed for good.")

It means forever, it's an expression.

It also means permanently. For example: “I’m giving up smoking for good” means I will never smoke again. We also use the expression “back for good” which means return forever.

It means definitively or forever

I think others have pretty well covered this, but I'll chime in (another expression you can look up). It can be literal, ex: "Why don't you stop your evil ways and work for good." But most of the time it mean PERMANENTLY. ex: "Why don't you break up with her, and be finished with her problems for good."