A mind map is a simple tool you can use to explore what concepts are related to a central concept, without having to do it all in your head. Putting those related concepts in a mind map format helps to keep them organised more than a simple list would.
To use this method, write down your main concept on a piece of paper or digital tool. Next, write any concepts that relate to that main concept around the central concept. Connect them to the main concept with lines. Next, consider each of those related concepts, and write down concepts that relate to those. Connected those to their "parent" concept with lines as well.
This method can help you map out a space that you need to design for, and might be useful when you need to come up with an information architecture, for example. It can also help you to consider concepts related to a problem, to see if those concepts inspire you to come up with new solutions.
Georgia was just hired by a new company that wants to create an online dating website. She's the senior designer, so it's her job to come up with the overall structure for the new website.
So she has some initial ideas to discuss with the team, she pulls out a large piece of paper and writes "Online dating" in the centre of it. She considers what concepts are related to online dating, and writes down "Messages", "Profiles", "Photos", "Matches", "Search", and "Recommendations" in a circle around the centre. She links them to "Online dating".
This is only the start, but it helps her to think about the major sections and features that her team will need to consider for the new website.