Indefinite integral or an antiderivative is a topic which you need to practice, practice and practice .
The basic formula involved :

intf(z)dz= F(z)+c, where c is the constant of integration.

Importance

This is a topic which is not that important if JEE-Advanced is what you have in mind but if JEE-Mains is what you aim for, then this is a topic with which you must be thorough.

As you can see from the trends, questions exclusively from Indefinte Integration are rarely asked in JEE-Advanced but there are chances that you may land a question or two in JEE-Mains.

But, applications of the concepts of Indefinte Integration have repeadetly been asked in JEE-Advanced, like in to order to solve a Differential Equation, you just cannot make do without the knowledge of Indefinite Integrals. Questions are hard to solve if you are not thorough with the concepts and the approach to a particular kind of solution.

Topics

1. Substitution: This is the most important part in the chapter.
2. By- parts: It's a simple formula, remember it. Also keep in mind that the ILATE rule is not always true.
3. Partial fraction.

Be thorough, practice hard- the two key words for the chapter. Also, be aware of the fact that you just cannot leave this topic out of the loop. Questions may not come directly from the chapter but its "applications are vast and frequent".