What are highlighters on a Kindle for?

Bryce Akins
Bryce Akins

In this book I'm readin (Lincoln Lawyer if it matters) and it keeps underlining stuff and says "show number of highlighters" and I don't know wht that's for or what it means

Tom333
Tom333

It highlights the text, so people interested in phrases, or people studying books can use it on their kindle. You might be pressing the highlighter button my mistake?