By SEO (Search Engine Optimization) you can optimize your sites search engine ratings. This will affect for example how the page is found in Google. But remember, the most important thing in search engine optimization is your site's content.
You can open the SEO section by clicking More on the Page info.
SEO options can only be seen if you have the SEO upgrade to SOLU.
Title
SEO Title is the title of your page that is shown in web browsers window tab and in search engines. You should keep the title optimized and not try to put too many keywords in it. Title describes the content of the page and every page should have a different title. Search engines will emphasises the beginning of the title.
Description
SEO Description is mostly for users that will search the content of the page in search engines like Google. Description field is shown in search engines and by that field users commonly decide if the page contains the information they are searching for.