Self-Discipline / Tapas
"Tapas means "warmth" or "heat" or "discipline/austerity".
In order for the yamas and niyamas to be effective in one's life, self-discipline is required. Discipline is remembering what we want and acting accordingingly Getting a clear intention with our lifeforce to follow it. Replacing negative habits, thoughts and actions with positive ones. For instance you might want to lose weight and become healthier. This may take some discipline to control yourself from eating certain foods.
Without self-discipline, one's actions, words, thoughts, become scattered, and the life-force/prana becomes weakened.
Self-discipline burns away impurities and kindles the sparks of divinity.
A. Chandra Bose explains the word as having positive significance, "not self-mortification but self-awakening by activising the spiritual power within oneself". For tapas refers to that contemplative act which as a result of contraction or focusing to one point (ekagrata) arouses to action the 'flame divine' or supreme creative energy, elsewhere in the Rgveda personified by Agni..."--G. Feuerstein, "the essence of yoga".