The Tooth, The Whole Tooth, and Nothing but the Tooth
In Room, Tooth is Ma's Bad Tooth, a rotten tooth that comes out inside a bagel one morning at breakfast. Old Nick doesn't exactly provide Ma with quality dental care while she's his prisoner inside Room. He won't even give her cream cheese for her bagel.
Jack saves Tooth, even putting it into his sock when he escapes from Room. He doesn't think of it too much when he's with Ma, but after she attempts suicide and Jack has to live with Grandma, he thinks about Bad Tooth almost every day. He holds it in his hand or his mouth like it's some sort of worry bead. Eventually, it becomes just "Tooth" instead of "Bad Tooth," because it's the only way Jack has to connect with his mother when she's in the Clinic without him. It's a bodily connection between Jack and Ma, in a way, and it might remind us that Ma has been breastfeeding Jack for all five years of his life.
When Ma returns, Jack realizes that he swallowed Tooth and lost it forever. Jack goes into a panic after losing it. He tells Ma that he has to have Tooth, but she says "Trust me, you don't" (5.904). Jack is left wondering if he'll ever poop Tooth out or if "maybe he's going to be hiding inside me in a corner forever" (5.908).
The latter is pretty much true. Tooth is a part of Ma, and whether or not she and Jack are together, he will always carry a part of her with him, literally or not.