Especially in greetings, people may respond "pretty good" and it may not be a very accurate reflection of just how well they are doing. ;-) Nearly always, people respond to "How are you?" with responses like "fine", "good", "pretty good", "doing ok", etc. "Pretty good" generally means good, or slightly better. But in conversation, the inflection, tone, body language, and context will tell you much more. The phrase itself is relative (so the real meaning just depends on your perspective). What's "pretty good" to me may be completely different for you. Using the word "pretty" like this (as an adjective/adverb) is frequently used with many words... pretty good, pretty neat, pretty expensive, pretty old, pretty rude, etc. In these cases, according to the Cambridge Dictionary, this means "quite, but not extremely". Note we cannot use "pretty" in negative sentences. We cannot say "The food was not pretty good".

In Portuguese (I noticed you're Brazilian) I would say "bastante bem".

Extremely Very Really Pretty Somewhat Ok I think I categorize my "good" qualifiers in that order.