打坐 is a separable verb (离合词) that specifically refers to sitting in meditation, a practice rooted in Buddhism and Daoism. Unlike 坐 ('to sit'), 打坐 implies a formal meditative posture with crossed legs and focused breathing. You can separate the syllables to insert duration or frequency: 打了一个小时的坐 ('meditated for an hour'). It's a cultural practice term — you wouldn't use it for just any sitting.