If you want to learn something, you need to be able to explain it, to everyone, in simple words so that they can understand, regardless of their cultural and technical background.
Teaching something, with simple terms, requires knowing it at its deepest meaning. Taking such step is important to go from just knowing something to being confident about it.
To fully do it, you need to explain it to someone who has absolutely no clue about the subject. If you cannot transmit your knowledge to a young student or your grandma it means you cannot synthesize what you know into simple, understandable statements.
Take it as a mental game first. Picture yourself while explaining something you want to learn to a virtual group of students, or a friend. They don’t have a clue on what you’re going to tell them. You explain them everything from ground up, with words that are as simple as possible.
This is much more than a mental exercise: it forces you to organize your knowledge into small bits of information, externalize it using words, and then rephrasing and refreshing concepts that would otherwise be static and inevitably settled in your mind.
There are also benefits doing that. Picturing yourself in front of a virtual audience will train you to explain your stuff and points of view to a real audience - be it your boss, a friend of yours or an entire classroom. And of course, if you’re not comfortable with the virtual classroom - or have no real person who wants to listen to you - you can write an article instead as an exercise. Maybe you can even start a blog, if you love sharing your stuff like I do.
The more you learn to transmit informations efficiently, the better you will assimilate that knowledge.