When you quote or paraphrase the work of another person in your research paper or presentation, you must provide a proper citation for the source.
A citation includes all of the pertinent information about a source such as it's title, the author's names, date published, publisher, and so on. The specific components for a citation depends on the type of source and the style used.
There are many different citation styles such as APA, MLA, Chicago, and AMA. Each discipline or field of study usually designates a specific style to use.
Citations are important because they give credit to the original author(s), enable others to locate the source that you cited, and improve the credibility of your work, especially if you cited authoritative sources.