A rating scheme is highly subjective; you can assign stars any way you want. Here are some general guidelines:
1 star: The photo is something I don’t want to immediately delete. Anything unrated is ripe for removal, so a single star represents my baseline.
2 stars: The picture has potential, is in focus (if that’s important to the shot), and is worth keeping. I also use two stars to mark photos that are worth revisiting in subsequent review passes.
3 stars: The shot is definitely a keeper and is worth touching up as needed in the Editor. Often a three-star photo will become a four- or five-star photo after editing.
4 stars: The photo is something I’m happy publishing online or sharing with friends.
5 stars: I want to see this picture every day of my life, or it’s something to revisit on those days when I feel like I can’t capture anything worth viewing.