This is probably a bit of a common mix-up, but it's so easy that if you haven't tried it, it's perfect for a weeknight meal.
It's a mix of:
1. A can of black beans (or pinto, the black just looks nice here)
2. A small bag of frozen corn (I zap it for a very short bit to thaw)
3. Small red bell pepper diced
4. 1 - 2 avocados diced
5. Juice of one lime
You can mix it up! Try jicama, tomato, chilies, and if you like it, cilantro.