While a common question, this isn’t exactly the best framing because a periodontist is a dentist, only with extra training and specializations.
Think of it as the difference between a general practitioner physician and a neurologist - both are medical doctors, but one has increased training and knowledge in a particular field.
Both dentists and periodontists are integral to maintaining and improving your oral health, but they have different focuses and will address different issues - although there can be some overlap with certain treatments.
Hope that helps you have an understanding of the periodontist meaning!