This is the 3rd Niyama and it is defined as self discipline.
The literal definition of tapas is Heat.
In this case the heat that builds from effort.
You can see tapas in action during your physical practices.
There is a heat that builds as you move.
When the tapas build, they purify the body.
Tapas are at work with any task that you want to perform and see results.
Because tapas are a repetition, to do something over and over again is necessary to see results and to succeed.
An example is when you are cleaning something, you wipe it over and over again until you get it clean.
Tapas help us to make healthy changes in our lives.
It’s important to practice self discipline and to have practices such as yoga that we do everyday.