Tapas means austerity or penance. There are many rules and definitions in the Vedas which apply here, like rising early in the morning and taking a bath. Sometimes it is very troublesome to rise early in the morning, but whatever voluntary trouble one may suffer in this way is called penance.
Similarly, there are prescriptions for fasting on certain days of the month. One may not be inclined to practice such fasting, but because of his determination to make advancement in the science of Kṛṣṇa consciousness, he should accept such bodily troubles which are recommended. However, one should not fast unnecessarily or against Vedic injunctions. One should not fast for some political purpose; that is described in Bhagavad-gītā as fasting in ignorance, and anything done in ignorance or passion does not lead to spiritual advancement. Everything done in the mode of goodness does advance one, however, and fasting done in terms of the Vedic injunctions enriches one in spiritual knowledge.