Raw Meat will give you 1 hunger point if you have it in your hand and you right-click. This will also make you poop. Pooping doesn't do anything, it's only visual. Raw Meat can be cooked at a fire made by a Firestarter by right-clicking while it's out in your hand in front of the fire.
You can get Raw meat from killing just about anything. The only things that won't drop Raw meat are chickens (they drop raw chicken) and villagers. Passive mobs (pigs, sheep, etc.) have a higher chance to drop Raw Meat.