Traits are rolled modifiers on a weapon. Three rules matter more than the unpublished effect list.
Rerolls change only the weapon you have equipped. The other slot is left alone, so you can test the off-weapon without wiping the carry.
The game asks you to confirm before you replace a Mythic or higher weapon, and before you reroll a Legendary or Exotic trait. Those two grade names are the ones in the confirm dialog. Individual trait names and their percents are not published. There is no auto-reroll. A failed reroll is reported to tell you why. Read that line.
You cannot farm rerolls in the current build. They were also removed from AFK chamber rewards on October 2. Sources that still pay them are codes, such as PATCH2's 3 Trait Rerolls, and quest rewards. Redeem codes the day they appear, and do not spend a reroll on a weapon you plan to replace this week.
Open the live trait panel before you spend. An older trait write-up overlaps early-access footage from a different Roblox place, so this page does not copy trait effects from that footage.