One is practice in managing thoughts and feelings. If you just sit and breathe, you'll begin to notice how much the mind wanders, how much thoughts randomly arise and pinball around your head. Meditation gives you a lot of practice in not getting caught up in that.
The other is what I think of as self-instrumentation. As with any sort of performance tuning or debugging, the foundation is paying attention to what's actually going on. Meditation has greatly increased my ability to introspect, which helps me solve a lot of things that in the past were mysteries to me.
Is there any theory of how meditation offers evolutionary benefits?
The book Mindfulness in Plain English is a great introduction to mediation if you are curious.
But, a quick, short answer - meditation is like going to the gym for your brain. In meditation, you practice detaching from thoughts. A practical example: Let's say you have a rocky relationship with your boss or a colleague. Maybe they say something you perceive as a slight while grabbing a coffee in the morning. Instead of fixating on it all day and letting it disturb your work, you just let it go. Not attaching to our thoughts doesn't mean we can't focus very hard on things - in fact, letting your negative thoughts go is what is going to allow you to focus hard on your work the rest of the day!
(Also, there are many types of meditation, this is just one form of meditation... again, lots of nuance here that is hard to get across in this format.)
That said, reading about meditation is never going to get you a full understanding of it, any more than reading cookbooks will give you an understanding of a cuisine you've never eaten.
Right. And do you really even need a whole book to explain how to do meditation?
Can it not be explained in one paragraph?
Is it even as complicated as a recipe in a cookbook with many steps and ingredients?
How many steps and subtasks and ingredients are there to do meditation?
Now I would be interested in seeing a book that explains some rational as to why a given meditation technique does some good to you. I believe it does, but don't know why. Therefore could it be like placebo?
Edit: Of course you are correct that the actual technique is very simple and can be explained in a few sentences. But there is much more to it than that - how do you process the results? That's what you need a book/teacher/help for.
You need many books to explain how to do meditation, because people vary widely and it's a complex set of skills with subtle goals. However, reading a thousand books is not sufficient to understand it, any more than reading all the books about bicycles can make you able to ride a bike.