It is to regard the conclusion of the proposition as a condition and the condition of the proposition as a conclusion.

For example, the condition of the proposition that the diagonal lines of parallelogram are equal: quadrilateral is parallelogram.

Conclusion: The diagonals of this quadrilateral are equal.

Its inverse proposition: a quadrilateral with equal diagonals is a parallelogram.

If you don't understand anything, you can continue to ask and hope to adopt it!