Namespace is the part of the name of the page before the first ":". The typical page has no ":" in its name, and is thus said to be in the mainspace.

Some namespaces--native ones--are automatically created by the MediaWiki software while others can be created by the administrators of the particular wiki site. The native namespaces include "Talk:", "User:", "Category:", "Template:", and others; they total 18 namespaces.

When searching the wiki, you can restrict the search to a particular namespace.