A budtender is like a bartender but for the dispensary counter. However, Budtenders are more than just shop clerks. A good Budtender is deeply knowledgeable about California cannabis generally and about the strains and brands on offer to you at that location.
Budtenders can help explain to you how to shop for weed and what your options are. They can ask you questions to see what you are after and can make tailored recommendations.
Budtender is a term from the legacy culture. Not all dispensaries use this term.