Mainspace refers to the main contents of MEpedia--the pages describing the science, history and people of ME. These pages appear in the MEpedia search bar simply by their names (without any prefixes) and they are also indexed to be searchable on web search engines.
Mainspace is distinct from pages with the prefix "MEpedia:", "Template:", "User:", and so forth. These pages generally deal with task of building MEpedia. For instance, projectspace refers to all project documentation pages, which begin with the prefix "MEpedia:". Userspace refers to individual contributor's pages and subpages.