The bus is an abandoned one on the Alaska frontier where the subject of the book, Into the Wild, Chris McCandless spent his last days on Earth. He wasn't prepared or trained to survive through an Alaska winter, and when he got injured (or sick, it wasn't completely clear) he was trying to survive by eating whatever he could.
McCandless dies of starvation in the bus. The cloth belt the author finds when he visits the site must have been made by McCandless, by hand, while he was laid up in the bus, in order to keep his pants up on his rapidly thinning frame. The belt, therefore, is evidence of the grim reality of his ordeal, and the time it took for starvation to play out.