Everything in this universe is energy, we in yoga believe that human being is the microcosm of the macrocosm, the macrocosm being the five elements in universe Viz. Earth, Water, Fire , Air and Ether. All these five elements are also present in the human beings, we constantly balance our body energy with the universal energy.
When these two energies are in synchronisation one feels the bliss in the body, in the mind and in the sprit. Any imbalance in these energies result in diseases which affects the body and the mind.
The physical postures which an yoga practitioner does on the matt is tuning the energy in the universe, Ex. A practitioner doing a head stand is defying the gravity and can stand upside down and maintain the stillness within the body and the mind. When we do the poses on the matt, we stretch, strengthen , integrate , expand the bodily energy attuning it with the universal energy.
Change is the only thing which never changes, yoga helps to accept the changes of the environment by understanding these energies properly.
The practise of meditation is not mere sitting in silence, the real practise is to understand the inner energy and to control the fluctuating thought process of the mind.