The iconic little lad in the image above is Vault Boy, the mascot of the Vault-Tec corporation in the world of Fallout
. You'll see him splattered in various parts all around the game. In advertisements, manuals, training films and products of the Vault-Tec corporation. He's also used to represent stats and skill trees on your trusty Pip-Boy.
A lot of the times, you'll see Vault Boy giving you a thumbs up, just like in the image above. Now, he's not really giving you a thumbs up, cheering you on or appreciating you. In fact, he's actually trying to determine whether he's in a radiation zone. Allow me to explain. When an atomic bombs go off, a mushroom cloud is formed.