What “Walk good” means
Walk good is a warm way to say goodbye that wishes the other person a safe journey. It's like "take care" or "safe travels", and it doesn't matter whether they're actually walking, driving or flying.
Examples
Walk good, mi fren
Safe travels, my friend.
Walk good
Take care / Safe travels
How to use it
Said as someone leaves, by anyone to anyone. It's kind and caring, so it suits elders and strangers as well as friends.