It is always a good idea to set your SEO site and page titles as well as descriptions. While descriptions do not necessarily help in ranking, search engines do use titles. In the Website Builder, there are three places to set SEO: Site Settings, Page Settings, and Site Overview (for bulk updating page SEO). These are for global site SEO terms and page specific SEO terms, respectively. Here is an example of how to set page titles and descriptions for your site.
Note
These are general recommendations. For personalized advice and more specific information, contact an SEO expert.
Titles (Site & Page)
The title tells users and search engines what the site or page is about. Make sure titles are clear about what users should see on the site or page. This is one of the most important steps, as the title is directly what shows up in search results on search engines. Also make sure not to repeat keywords in titles, as this can be seen as trying to spoof results. A good page title should be no longer than 70 characters.
Description
The description is what shows below the site's or page's title on search engines, though some search engines, such as Google, will not always use the set description. Depending on what the visitor is searching for, the search engine may provide a different description. A good description is between 130 and 160 characters and tells an accurate description of what the site is about. Here is an example description for a mobile website builder:
The fastest, easiest way to build a great looking mobile website in minutes. Includes hosting, site analytics, click-to-call, mobile maps and more
This description is 146 characters and accurately describes what the service does.
Keywords
Keywords are the terms that people search for while trying to find your site, though most search engines no longer take meta tag keywords (what is added in the site or page SEO settings) into consideration when ranking websites, so these are optional. The important part about keywords is that they match the content on the desktop site. Meaning that if you are running an oil change station, you should not have keywords relating to sushi. Here are the Keywords that CNN.com uses:
CNN,CNN news,CNN.com,CNN TV,news,news online,breaking news,U.S. news,world news,weather,business,CNN money,sports,politics,law,technology, entertainment,education,travel,health,special reports,autos,developing story,news video,CNN Intl
Notice that the keywords relate exactly to what CNN does. Also, they do not repeat CNN in every single keyword. CNN could have easily put CNN news travel, CNN news autos, and so on, but they know that repeating keywords is not a best practice.
Further Reading
If you want to learn more about SEO best practices:
Getting started guide released by Google (only available in English)