This is a string which is (usually) put between words of the language. It is used, e.g. when messages are concatenated (appended to each other). Note that you must express a space as html entity   because the editing and updating process strips leading and trailing spaces from messages.
Most languages use a space, but some Asian languages, such as Thai and Chinese, do not.
The plain format does not allow any processing. This means the following features are not allowed: MediaWiki magic words (such as {{SITENAME}}), parser functions (such as {{PLURAL:}}, {{GENDER:}}, {{ns:}}, {{#special:}}, etc.), wikitext markup (such as '''bold text''', [[wikilink|text]], etc.), HTML markup (such as <span lang="en">foo</span>), HTML character entities (such as &zwsp;, {).
Due to limitations in the software using this message, only three very specific HTML character entities allowed even in plain mode; they are:
  (regular space) (non-breaking space, named)  (non-breaking space, numerical)