Sometimes you don't need to save/reuse an entire section to your library, all you want is a small blurb of text, maybe only a couple of paragraphs long.
For example you might have an intro paragraph on your overview section that you want to save seperately. That's where Snippets are useful.
To create a new snippet you can do one of two things:
Go to content library > Snippets and click the new snippets button. Give it a name, add your text and tag it if you want.
Or when you are editing a proposal or template, highlight the piece of text you want to save, click on the snippets tool and click "+ Snippets" button. Then name your snippet and it's saved to the library.
Once a snippet it saved to the library all you need to do to use the snippet is to make sure your cursor is in a text box, click the snippets tool, and find you snippet from the drop down. As soon as you click the snippet you want it gets inserted in your text box.
Changes you make to the snippet once it's inserted won't affect what's in your library, it gets saved separately and just becomes part of your proposal.