Go to "Settings" (on the top of the builder);
Go to "SEO". There you can specify:
"Title" (visible as a browser's tab name for every page); It is the most important for search engines;
"Description" is set to advise to a search engine what every page is about;
"Keywords" are still used by some search engines, but Google ignores them (write keywords in line, separate with commas or semicolons, spaces between words are not important);
"User-friendly URL" is the optional field if you want to specify a custom URL path to a page. The default value is taken from the page name specified in the menu.
See how to add custom meta tags.