A Trait streak is when a card that is upgraded has all of the levels the same colors ie all their traits match. 

For Forge 24 the different traits in a trait streak are

1. A card with a Bronze name plate, frame and background a card with 
2. A card with a Silver name plate, frame and background
3. A card with acard with Gold name plate, frame and background
4. A card with a card with Diamond name plate, frame and background.