Traits, in the context of Fallout, are character modifiers that provide a significant advantage of one form or another, at the cost of a significant disadvantage. The “Heavy Handed” trait in Fallout: New Vegas, for instance, grants characters a 20 percent bonus on melee and unarmed damage, but also imposes a 60 percent penalty on critical damage. Character traits are are an optional, but for many players vital, part of character creation in Fallout, Fallout 2, and New Vegas—but Bethesda chose to omit them Fallout 3 and, more recently, Fallout 4. Now, thanks to the work of a modder by the name of Cirosan, it's possible to bring them back.
The “Traits and More Perks” mod that hit the Nexus over the weekend adds ten Traits, and a few new Perks, to Fallout 4. Many of them are inspired by traits from previous Fallout games—“Heavy Handed” is present, for instance (with slightly changed stats)—but others, like "Determined" (permanent bonus to damage, energy, and rad resistance, limbs are harder to cripple, but chems have no effect) are original creations.
Because of the way Fallout 4 is designed, the system for acquiring Traits is a little different than usual. Instead of adding them during the character creation process, you'll need to construct a special item at a Chemistry Station, and then use that item on your character. You can have as many Traits as you want, and you can also create items that will remove all Traits and mod-made Perks (Perks that were part of the original game won't be affected), so you can re-spec your character if things aren't working out.
The mod works with existing saves as well as new games, and Cirosan said he's playtested it thoroughly but cautioned that it's still in beta, so there's a possibility that something might not work quite as intended. The comments seem quite positive, however, although there's apparently a problem with applying traits in the third-person view, so make sure you're in first-person when you do.