Hathayoga is one of the oldest yoga forms. Many modern
physical yoga forms originate from it. As in all other yogaform there is a combination of movement, relaxation and breathing.
Hathayoga is a soft, but physical yoga. It is a series of positions where you move from one to the other in a longer or shorter sequence. You probably recognise some of the yoga positions (asanas), the warrior II, the downward-looking dog, the cobra position, etc. The movements between different positions are combined with breathing and the class ends with relaxation and a breathing exercise.
For a hathayoga teacher it is important to understand anatomy and limits and possibilities for each individual. Everyday Yoga School where I did my teacher training focus a lot on these topics.