So, imagine you're playing a game and you're up against a really big, scary robot. It seems like there's no way you can win because the robot is so much bigger and stronger than you. But then, all of your friends come over and they all have their own tiny robots. Individually, they don't seem like much, but all together they make up an army.
So, "An Army of Davids" is a term used to describe a situation where a large group of people, who individually may not seem like much, come together to take on a big challenge or problem. It's like when lots of little guys band together to take on the big guy.
This idea was first put forth by a man named Glenn Reynolds, who wrote a book about it. He believes that with the help of technology and social media, lots of little guys can come together to do big things. For example, people can use Twitter to organize a protest against something they don't like, or they can use crowdfunding sites to raise money for a good cause.
So, think of it like this: One person might not be able to change the world, but if lots of people come together, they can accomplish amazing things. And that's what "An Army of Davids" is all about.