In most templates, the description field is populated from the first paragraph of your page content. By adding a pragraph break, you can shorten what appears there.
Having said this – the more important question is: what do you want to achieve by shortening the description? This field was originally designed to help search engines index your page, but nowadays it is mostly ignored (Google), or only checked if it actually exists (Bing). For this reason, putting in much effort in editing your content, just to get a "better" description is not very useful.
There are still some other use-cases, like my own "Ad Hominem" template uses the description to provide a link preview (i.e. if you hover the mouse pointer over an an internal link, it shows the title and the description of the target page as tool tip). However, if you don't use that template, well, there is not much of a point in it either, really...