Tapasya means restraining senses through knowledge of one's eternal nature and devotion to Krishna.Listen — Srila Prabhupada Uvaca
Prabhupāda: Oṁ namo bhagavate vāsudevāya. [devotees repeat] Texts number thirteen and fourteen. [leads devotees in chanting] tapasā brahmacaryeṇa śamena ca damena ca tyāgena satya-śaucābhyāṁ yamena niyamena vā deha-vāg-buddhijaṁ dhīrā dharmajñāḥ śraddhayānvitāḥ kṣipanty aghaṁ mahad api veṇu-gulmam ivānalaḥ [SB 6.1.13-14] [02:29] So, how to become advanced, these are the prescribed methods. Tapasā.
Tapasā... Tapa means voluntarily taking some disadvantage. That is tapa. Tapa.
Tapa means, just like if there is fire, so there is heat. But the method of tapasyā is that during summer season they ignite some firewood all round and sit down. Already there is scorching heat, and still, all round fire, and one has to sit. These are the some of the examples of tapasyā.
Similarly, in the winter season it is very cold: one has to go down the water up to neck. This is the meaning of tapasyā: voluntarily accepting some severe condition of life. So in this age it is very difficult, but this is the meaning of tapasyā, voluntarily accepting inconveniences. When there is cold, one has to take the help of heater, fire.
No. No heater, no fire, but go deep into the cold water. Of course, it is very difficult in your country, because the water is so cold, and if you go deep, immediately finish. [laughter] I have seen in New York.
One dog, he jumped over—immediately finished. I have seen it. That is actual fact. So Caitanya Mahāprabhu is so kind and..., that these people in this age, they will not be able to undergo severe austerity. That is not possible.
Mandāḥ sumanda-matayo manda-bhāgyā hy upadrutāḥ [SB 1.1.10]. In this age every man is alpāyuṣaḥ. Alpāyuṣaḥ means very short-living. The limit is hundred years, but who is going to live hundred years?