De architectura is a really old book written by a guy named Vitruvius. In the book, he talks about how to build things like buildings, roads, and even machines.
Imagine you're playing with building blocks. When you're building a tower, you want to make sure it's not going to fall over. Vitruvius talks about how to make sure buildings don't fall over, too. He talks about using strong materials and making sure everything is balanced so it stays standing.
Vitruvius also talks about things like plumbing and heating. When you take a bath, you need hot water, right? Vitruvius talks about how to make sure there's hot water in the bath without the rest of the house getting too hot. He also talks about how to make sure the water goes away after you're done with the bath.
De architectura is really long, and it talks about a lot of things that people still use today when they build things. It helped people figure out how to build things that would last a long time, and it's still useful even though it was written a really long time ago.