Note: This may seem ridiculously fundamental, but no one ever explicitly told me what this was. That is partly this series goal, to address what young developers have to assume because it's "basic" or "assumed" or knowledge to be picked up.
A script is simply a chunk of programmatic code.
Sometimes, it can be helpful to think about the non-technical context of the word. A script for a theatre is a play-by-play about what is going to happen in a story. I find it helpful and less intimidating to assume code is trying to tell the whole story, and each line is a play-by-play about what will occur.