देवद्विजगुरुप्राज्ञपूजनं शौचमार्जवम्।ब्रह्मचर्यमहिंसा च शारीरं तप उच्यते॥
deva-dwija-guru-prājña- pūjanaṃ śaucam ārjavambrahmacaryam ahinsā ca śārīraṃ tapa ucyate
Word by word
The verse says
Respect shown to the bright powers, to teachers, to the learned and the old; cleanness, straightness, restraint, not harming - this is called the discipline of the body.
In plain words
Tapas is usually translated as austerity, which sounds like suffering. Look at what is actually on the list.
Showing respect. Keeping clean. Being straight with people. Holding back. Not harming anybody.
None of that is extreme. It is only hard to keep up, which is a different thing, and the harder of the two.
Where you've felt this · in parenting
You get your son to wash his hands before eating, every day, for eleven years. There is no day on which this is dramatic. There is no day on which he thanks you for it. Then he is twenty-four, living somewhere else, and he does it without thinking. Nothing in that was difficult. All of it was relentless.
Going deeper
Discipline is redefined here as something ordinary and continuous rather than as suffering, and the list is the argument.
Tapas literally means heat - the heat of effort. People hear that and picture extremes. What is given is cleanliness, straightness, respect for people who know more than you, restraint, and not harming. Every one of them is unremarkable on any given day.
What makes them hard is that they do not end. An extreme act has the advantage of being over. These do not finish, and they produce nothing to look at. Nobody has ever been admired for twenty years of washing their hands.
That is the real reason the word is heat. The burning is not in the difficulty of the act. It is in doing an undramatic thing long past the point where it is interesting, and people who can manage that turn out to be able to manage most things.