The little icons in your pipboy above the dps/damage. Its like a little icon with a number next to it. Its when you highlight weapons.

The icon tells you what weapon category (guns, energy weapons, explosives, melee, unarmed), and the number tells you the skill point threshold to use the weapon effectively.

thank you

It's the skill you need to use the weapon. The number is the minimum rating you need to use the weapon effectively.

Lightning bolt = Energy Weapons
Bullets = Guns
etc.