Citations document source information used in research. They add credibility to your work by showing where your information came from and give proper credit to the source material.
Knowing where to find and how to read citations will also help you significantly with your own research by pointing you towards ready sources of relevant information.
When to document a source
As you gather information to use in your research paper, it is important to keep track of where you found it. Your work should include a bibliography of all the sources you used, which you will also reference whenever you use information from them.
You do not need to cite your own ideas or any information that is considered common knowledge.
Everything else must be properly credited, using a commonly accepted citation style.