References (or refs or simply reliable sources), on Wikipedia, are important to verify content and inform the reader of its source. Any editor can remove unsupported statements if they cannot be verified in a reliable source reference they can add to the article, and unsubstantiated articles may be deleted if no reliable sources coverage can be found. When adding content to an article, cite your sources to help readers verify facts or find more details. It is also useful to update or improve existing references.
This page shows you how to provide inline citations using ref tags, which result in superscripted footnote numbers, like this: and numbered references (footnotes) being added to the reference section near the bottom of the article. There are other acceptable systems, including the use of inline parenthetical references and general references. As a general practice, the first major contributor gets to choose the referencing system for that article.
There are automated tools such as Refill and Reflinks that can help to add references.