Tapas/tapasya (lit. heat) 1. Austerities. 2. The experience of heat that occurs during the process of practicing yoga.  The heat is generated by friction between the senses and renunciation.  It is said that this heat, called "the fire of yoga," burns up the impurities that lie between the seeker and the experience of the Truth.
On the list of niyamas is austerity, tapas, which literally means heat.  This is the fire of yoga.  First, it burns up the impurities in the body.  Then it reveals itself as the great fire, the blazing light of the supreme Self.  For a mediator, tapasya also includes accepting whatever happens as the best thing for your sadhana, and not being disturbed by any discomforts. inner and outer, that you may experience.

Austerity develops endurance, which is the backbone of yoga.  In yoga, your need the power of endurance.  Constantly enduring whatever happen, never falling apart.  You should never give yourself a chance to fall apart because when you do, it becomes a tendency, and it happens over and over.  Endurance is an invaluable quality that strengthens a mediator and helps him overcome the obstacles on the path.  The path is strewn with many obstacles, so you can't let one obstacle stop you.  You have many obstacles to overcome, so you must gain strength as you walk the path.

Even though at times you feel very tired and depleted, so tired that you feel you can't go on, even then, remember cleanliness, remember the purification that has already happened.  Remember that austerity is good.  It is the real fire that will burn the impurities and obstacles.  Do not let yourself become completely exhausted.  Without austerity, it is easy to give in to temptations and fall back on the indulgences that are so much a part of the lower self and the world of sense objects.  So a yogi, a seeker, must always remember to ascent higher and higher.  There should be nothing in the closet to revert to.   Many people think if they put their bad habits in the closet, somehow no one will see them.  Then, if they really want to indulge themselves, they can.  But it's better to keep the closet clean.

Austerity has the power to mold a person into a sturdy vessel capable of holding the immense energy of yoga.  This energy, this shakti, is very strong.  In order to hold it, your need a strong body, a strong vessel.  To sum it up, every part of your being profits from austerity, which curbs the appetites and purifies the will.