Yoga is many things, many concepts and usually something different to each practitioner.
At the start of class, I sometimes ask my yoga students to think of all the contrassts or opposing actions/sentiments/ aspirations we experience in our yoga practice. These might include strengthening and relaxing, right and left, up and down, forward and back, movement and stillness, breath in and breath out. So what does Hatha Yoga mean? The word Hatha can be broken into two parts: Ha meaning Sun and associated with our masculine, energetic side and Tha meaning Moon, associated with our feminine, passive, relaxing nature.
In yoga philosophy, the gunas encapsulate these concepts.
Rajas represents dynamic energy, effort, will. Whereas Tamas is passivity, inertia, potential energy.
We can endeavour to find a state of Satva – pure balance between the two in both our yoga practice and our life off the yoga mat.  That is, while finding just the  right amount of exertion and relaxation, we will become more open-minded, disciplined, patient, loving and authentic to our true essence.