Original: The "thief" mentioned in the song refers to the tumor. The short clip at the end of the song consisting of a young girl singing a Sunday school song is a real audio clip of Mina Kim singing "Little By Little" with Maida's wife Chantal Kreviazuk.
Simplified: The word thief in the song refers to the tumor and at the end of the song there is a sound clip of a young girl that is actually Mina Kim singing a "Little By Little" with Maida's wife Chantal Kreviazuk.