Better is a comparison word, meaning something is more acceptable, beneficial, of a higher quality, or more useful. You have to compare it against something else. Example: Eating fruits and vegetables is better than eating junk food for your health. The thing being compared can be understood or implied from the context: Eating fruits and vegetables is better for you [than eating other things]. The "than eating other things" is understood and assumed in the meaning of that sentence.
The word good is similar but doesn't use any comparison: Eating fruits and vegetables is good for your health. The word good also has the meaning of someone or something that is not acting bad or harmful. Example: This is a good dog (meaning he behaves well and obeys and is nice).