One of the key policies of Wikipedia is that all article content has to be verifiable. This means that a reliable source must be able to support the material. All quotations and any material whose verifiability has been challenged or is likely to be challenged must include an inline citation of a source that directly supports the material. This also means that Wikipedia is not the place for original work, archival findings that have not been published, or evidence from any source that has not been published.
If you are adding new content, it is your responsibility to add source information along with it. Material provided without a source is more likely to be removed from an article. Sometimes such material will be tagged first with a  template to give editors time to find and add sources before it is removed, but sometimes editors will simply remove it because they question its veracity.
This tutorial will show you how to add inline citations to articles, and also briefly explain what Wikipedia considers to be a reliable source.