Vimscript is a long and complex topic, and this chapter only brushes it. If you want to learn more, there are a few options.

You can read :help eval, which contains most of the information about Vimscript.

You can also choose to follow a tutorial online, or pick up a book. A lot of people recommend 'Learn Vimscript the Hard Way' by Steve Losh, and it is indeed rather good. It's available online at http://learnvimscriptthehardway.stevelosh.com/ (you can buy a paper copy from the website as well).