Adult dental is not one of the ten essential health benefits, so most ACA health plans do not include it. That is the single most common gap people discover at the worst possible moment — a cracked tooth is not covered by a health plan, and a crown is not a small bill.
A standalone dental plan is inexpensive relative to major medical and generally follows a simple structure: preventive care covered at or near 100%, basic work such as fillings covered at a set percentage, and major work such as crowns and bridges covered at a lower percentage, usually after a waiting period. Pediatric dental is an essential health benefit and is already included for children on ACA plans, so this is primarily an adult purchase.
Dental plans can generally be bought year-round, which makes them one of the few useful moves available outside Open Enrollment.