If you are not familiar with mind maps, they are way of connecting ideas. You start with a single concept in the center of a page and then add in connected ideas. Each idea is typically called a node, and each node can connect to more new nodes or to other existing nodes.
If you created an NPC their name could be a node and then as you decide things like hair color, gender, name, and location these could all be child nodes. As you talk to them a learn more facts you can add nodes to the map.
These are a great way of seeing connections at an overview level.
What you can end up with is something that looks like a police investigation with all your suspects, which obviously fits in with using these in investigative games.
RPGs can be very linear. This means that your character may have learned or seen something hours ago in their world, days or weeks ago in real life, but it nearly registered in your game at all.
The mind map lets you capture those ideas or little facts in a quick and easy format, that is definitely non-linear.