Okay, so imagine you have a library full of very important books that have a lot of good information inside. Now, imagine that some people can't afford to buy these books or don't even know they exist. That's not very fair, is it?
Open access in Germany means that people can access these important books and information for free, online. This way, everyone can read them and learn from them, no matter where they come from or how much money they have.
It's like the library is open to everyone all the time, even if they're not in the same city as the library. This is possible because some really smart people have figured out how to put everything that's in the library online, so it can be accessed from anywhere in the world.
This way, people who can't afford to buy books or who don't live near a library can still learn and grow from the knowledge that is available. It's a really good thing because it means that everyone has a chance to learn, no matter who they are or where they come from.