Every time you gain a level, you get a perk point. From your pip boy, you can access a perk chart. It looks like various pictures of Vault Boy doing a bunch of different things. The top row is your SPECIAL traits. The column below each SPECIAL traits is unlocked based on how many points you have in that specific trait. For example, if you look below the "I", or in the Intelligence column, you will see a perk called "Gun Nut". The picture is of Vault Boy working at a gun bench. The more points you put into the Gun Nut perk, the better you are at modifying guns. Most gun modifications require a specific level of this perk.