This is a phrase used to help differentiate between the similar-sounding words "do" and "dentist." It is a type of minimal pair, which are pairs of words that differ by only one sound but have different meanings. The phrase "do or dentist" is used to practice pronouncing the // and /d/ sounds in English.
This phrase is important for clear speech and communication. The // sound (as in "do") is a voiced dental fricative, which means that it is produced with the tongue against the teeth and the vocal cords vibrating. The /d/ sound (as in "dentist") is a voiced alveolar stop, which means that it is produced with the tongue against the alveolar ridge (the bony ridge behind the upper teeth) and the vocal cords vibrating.