Thaat is the Hindustani classification system for parent scales. There are ten thaats in the system codified by Pandit V. N. Bhatkhande in the early 20th century: Bilawal, Kalyan, Khamaj, Bhairav, Bhairavi, Asavari, Todi, Purvi, Marwa, and Kafi. Each thaat is a heptatonic scale — seven notes — and each has a characteristic note pattern.
The thaat system organises raags by which notes they use. A raag in Kalyan thaat will share its underlying note set with Yaman, Bhupali, and Hamsadhwani. But these are different raags because they emphasise different notes, follow different aroha/avaroha patterns, and have different characteristic phrases.
Carnatic music uses a different parent-scale system called melakarta, with 72 fundamental scales rather than 10 thaats. The two systems are not directly interchangeable — they reflect different historical lineages of South Asian classical music.
In practice, the thaat is the listener's first orientation point. Once you hear which thaat a raag belongs to, you can predict its general note set and narrow down which specific raag is being performed.