In Bhagavad Gita it's mentioned that Yajna, Tapa and Dana are the Karmas which a person should never abandon in his life.
What is 'Tapa' in Hinduism?
Yes, the Bhagavad Gita says it in the following verse:
Tapa generally means austerity like meditation, self control and other spiritual practices. The 17th chapter of Gita states the different types of tapa. They are: Satvic, Rajasic and Tamasic.
1. Satvic Tapa (Austerity in the mode of goodness)
It is of three kinds, austerity of the body, of the speech and of the mind.
The threefold austerities in the mode of goodness are done with faith without any personal desires. [BG - 17.17] But the other two types of austerities are tainted with personal desires.
2. Rjasic Tapa (Austerity in the mode of passion)
2. Tamasic Tapa (Austerity in the mode of ignorance)
If you consider Life as an on going Yagya, where you sacrifice "things" to get "things", then Tapa is to put your own self, as an Aahuti or sacrifice, in that Yagya, so that whats left after is only PURE Self or Consciousness or Atman. (Remember that Agni purifies)
NOTE: I seldom provide references in my posts when I present my views on the subject, because I think looking up to a book every time you "think!", is a again NOT a hindu trait. Vedas were in souls of ancient sages, they did not mug it up either. Reading Vedas should be a journey of Mind to soul, rather than of memory from one synapse to the other.
Mind you, that there is a difference between Vedas and books of other religion. "Vedas" are NOT "Holy" as in "Holy Bible" or "Holy Quran", but they can for sure MAKE YOU HOLY.