This is the name of the default page in each namespace (including the root namespace,) which is loaded when no page name is given. It should be a valid pagename and never contain a namespace.
Typical other values are
home or a whatever
start is called in your native language.
Note: This documentation page is named startpage to avoid confusion with start page of the config namespace. The option is just named
Warning: Changing this option could cause unintended behaviour when changed in a wiki that used a different value for a while.