Meaning: This means to clean your hard, white structures in the mouth called teeth because they are not clean.
Choice of Words: "Brush" means to clean something with a brush, "teeth" are the hard, white structures in the mouth used for biting and chewing, "they're" is a contraction of "they are," and "dirty" describes something that is not clean. Together, the sentence tells someone to clean their hard, white structures in the mouth used for biting and chewing because they are not clean.