Many therapists running their own business or private practice opt to become an LLC (limited liability company). An LLC is a specific type of legal entity that grants you limited liability and makes you as an owner not personally liable for the debts or liabilities of your practice.
A PLLC is known as a professional limited liability company, or professional LLC, and is sometimes required for occupations in which a person is licensed by the state for the services they provide. This can often be the case for therapists in private practice. Each state has its own requirements and designations and may require you to form a PLLC in lieu of an LLC, depending on where you live and work. You'll need to research and work with the state you operate out of in order to determine which entity is best for and required of you.