Basically, a Mind Map is a diagram which you create yourself as a way to organize ideas. In conventional note-taking, you write down information line by line or perhaps column by column. Mind Mapping differs from such note-taking in that you present the information more in the form or a diagram, starting with a central key idea drawn in the center of the paper.

Other ideas which are somehow related to the central key idea are arranged radially around it, with lines branching out from the central key idea to these subtopics to show that they are related to one another. Details related to each sub-topic can be shown to be connected to it through more lines.
It looks something like the picture below.

mind map technique

Looking at this diagram, you can see that the keyword in the center is important because it is the main idea. We can also see that the Subtopics support the main topic and the Subsets support the Subtopic. Not only that, but we can easily see any subset of subtopic on the top left is unrelated to ideas in the top right, bottom right, or bottom left.

This is useful when you have muddled thoughts that need to be sorted out, or bits and pieces of information whose relationships to one another have to be visualized. When you use mind maps in this way, complex problems become simpler to think through and find solutions to.