What are partials? Partials are chunks of code that you will use multiple times throughout your site. The header or nav or even footer are the most common ones.
Rather than having to code these in every page you simply code them once. This also helps a lot if you need to change them after you have them in many pages.
How and when to use them. It makes sense to create a partial every time you think you may need to use this code in more than one place.