Scott gets two bands of differing widths tatooed around his left arm as a reward for not trying to talk to Allison all summer. He says that it's a pattern he draws with his fingers and traces a simple bullseye. This is later revealed to be the growth rings of the nemeton.
Somehow, Derek is able to use a blowtorch to keep Scott's tattoo visible. It is not explained how this is supposed to work, since it is clear that burned skin can eventually heal on a werewolf and wouldn't result in black lines. Nor is it clear how a burn would force the ink to the surface to remain visible.
The bullseye has become the symbol of Scott and the McCall pack in fandom.
Real World Significance
In real life, Tyler Posey got the bands tattooed on his arm without letting Jeff Davis or the show know beforehand. They had to invent a reason for Scott to have the tattoo so they wouldn't have to keep covering it up on the show.