The header plugin gives you a way to edit the tags that appear in the HTML header of your pages. These are generally not accessible while you edit the visible part of your page.
There are already many entries automatically generated by the Snap! Websites system that go in the header. However, certain things cannot be automatically generated, or can be better if entered by you.
For example, your headers can include a description meta tag. This description can be generated using the teaser code (in effect, using the first X words of your page.) However, in most cases, the description should be a very condensed version of your entire post and not just the beginning/teaser of the post.
Similarly, you can enter information such as a brief abstract or the location where the event mentioned took place (i.e. as with defining the longitude and latitude information of your event location.)
The header is also where some systems check for a specific meta tag to verify that you own your site (generally called "site verification".) The headers plugin helps you with that by allowing you to enter any tag you'd like in your website header.
It is also possible that you would like a certain header to appear on all pages. For example, you could enter a copyright notice and an author name that in the end gets repeated on all pages. This is also available in the global settings of the headers implementation.
The header plugin offers all those options by giving you the ability to fill out forms and save that data in the page concerned.