Symbolism, Imagery, Allegory
Whether it was the rabbit's foot you got out of a vending machine at the mall or your smelly socks you refused to wash because they'd get you an A on your math test, you've probably done the lucky charm thing at some point. The difference between your rabbit's foot and gym socks, though, and Sam's medicine pouch, is that his is a matter of life and death.
Tia made the pouch for Sam when he was a baby to protect his necromancer aura from being visible to others, especially Douglas. "Most medicine bags protect," she explains. "Yours was more like a shield" (14.11). So the medicine pouch symbolized protection from Douglas and the spirit world, as well as reminding us constantly that Sam's someone who needs this protection.
It's meaning goes deeper than this, though. In reality, it can also represent Sam's relationship with his mother and how, despite keeping secrets from him and being a little freaked out by his power, she's loved him all along. According to Sam:
My mom had made [the pouch] for me when I was really little and kept having nightmares […] She'd tied it around my neck, telling me never to open it because that would let all the magic out. (4.68)
Obviously, there was a lot more going on than just Sam having nightmares, but the pouch still reveals Tia's desire to protect her son from the evil around—and inside—him. And it makes it clear to Sam that while she may have avoided him physically over the years, she's always cared.