The Mac has a great built-in tool named Summarize, which does just that. If you have a chunk of text that is too long, then you can shorten it using the Mac’s very own TL;DR generator, a system service which will take any text and shrink it, keeping only the important bits.
Perhaps you want to skim-read a too-long text? Maybe you want to reduce a full article to a 140-character Twitter post? Or maybe you want to email this article to a friend of yours who is too lazy to read it, but could totally use the advice.