Paleo refers to the diet that humans are genetically adapted to eat. This is the diet of hunter-gatherer populations and was the way all human beings ate until about 10,000 years ago, out of our 2.5 million year existence, when agriculture began to appear on the scene.
Simply, this way of eating takes us back to the basic way we were meant to eat. You eat moderate amounts of grass-fed, high quality meats, wild caught fish, lots of local vegetables (all you want of the non starchy vegetables), limited amounts of seasonal fruits, and moderate amounts of healthy fats including grass-fed lard/tallow, avocados, eggs and coconut.
It is free of grains, legumes, salt, dairy products, and processed foods. It is a simple way of eating. The challenge is finding ways to eat like this in the maze of modern food traps placed before us.