Okay, so imagine you and your friends are playing make-believe, and you all decide to pretend to be soldiers. One of your friends, we'll call him Garibaldi, is really good at playing this game and he has a lot of cool ideas for what you all could do.
Garibaldi tells you all that he wants to lead you on an adventure. He says you're going to march all the way from one end of the playground to the other (it's a really long way!). But that's not all - you're not just pretending anymore, you're actually going to do it! Garibaldi has a whole group of grown-up friends who are willing to go on the adventure with you all.
So one day, you and your friends and Garibaldi and all the grown-up soldiers start marching. It's hard work, and your feet hurt, but you're all really excited because you're doing something big and important.
Finally, after many days of marching, you reach the other end of the playground. Everyone is cheering and clapping and giving you and your friends high-fives because you did it! You completed the adventure!
That's kind of what happened with the Expedition of the Thousand. A guy named Giuseppe Garibaldi decided he wanted to lead a big group of soldiers on an adventure to try to take over part of Italy. He had lots of people who were excited to go on this adventure with him, and they marched all the way from one end of Italy to the other (which was really far!). Lots of people joined in and it became a big deal. Finally, after a lot of walking and fighting, they were successful and took over the area they were after. People were really excited about it, just like you and your friends were when you finished marching all the way across the playground.