Seiza is one of Japanese customs. Seiza is known that it makes the posture of the spine properly. I think now many other countries teach seiza especially in martial art classes like judo.
Can you do seiza? No? It’s easy. First of all, kneel down knees on a floor. At this time you want to make sure that the big toes are NOT crossing; they need to be touched each other. Then lower your hip (butt) onto the heels. And straighten the back (and it is above the heels directly).
That’s the formal way of sitting in Japan. When you sit on a floor in Japan, it’s polite to do seiza. Once you get used to do it, you would like to do it often.