Once you know a Tai Chi form, then when you perform it, your mind should be fully engaged with the moment-by-moment action that your body is doing. This integration of conscious mind and body movement means that your mind is focusing on the NOW and is not worried or distracted. That is relaxing and good for your mental health.
A somewhat over-hyped term is ‘Mindfulness’ it has in the last few years become somewhat of a ‘in thing’, but mindfulness has been an integral part of Tai Chi for several hundred years.
The first phase of Tai Chi is learning what you are meant to be doing. What foot goes where? How you move your arms and body? Possibly, depending on the teacher, synchronising your movements with your breathing. It’s not ‘difficult’ (for most people) but like learning any new skill, it does take time and practice.
The next phase is the interesting one. Once you have learned a few moves, so you know the physical ‘what goes where’ of it, you start to practice paying detailed attention to what is happening – the placement of your limbs, your weight distribution, the depth and speed of your respiration, your state of mind and more… This concentration moment-by-moment on YOURSELF is a form of meditation and it is both deeply engaging and relaxing.
Also, the movements themselves are done in such a way that virtually no stress is placed on the body and that is relaxing too.