Paleo means eating the way humans ate thousands of years ago. Meat, fish, eggs, vegetables, fruits, and nuts. No grains, no dairy, no processed food, and no added sugar.
The idea is that modern processed foods are relatively new and our bodies are not built for them. By eating only foods that existed before farming, you remove most of what is considered unhealthy in a modern diet.
It is similar to keto but allows more carbs from fruit and starchy vegetables like sweet potato.