A mind mapping exercise is a quiet individual brainstorm session, where you quickly write or draw every thought in your mind with no specific order or formatting. The thoughts could be connected or could be not, could be useful or could be not. The key is to not overthink and lay out all the thoughts, which later on could provide a pathway or map to possible solutions.
After the research phase, there could be many thoughts and ideas in the designer's mind. Laying out ideas neatly one by one could take a long time and some ideas might get lost along the way. Mind mapping allows the designer to quickly transfer all thoughts or ideas from mind to paper.
In the Diverge phase, as a preparation for concept generation.
Mind mapping is a simple process. Grab a pen and a piece of paper, and simply start writing or drawing until you think all the thoughts are laid out on paper. Then you can review the thoughts and ideas on the paper, and try to connect them with lines.
References:
https://library.gv.com/the-product-design-sprint-diverge-day-2-c7a5df8e7cd0