A "citation" is the way you tell your readers that certain material in your work came from another source.
When we use another person's idea in our research, we must include a brief notation next to that idea to let our readers know who developed it. This brief notation is called an in-text citation. At the end of our work, we include a fuller notation, which provides details that allow others to identify and locate the source in which we found that idea. This fuller notation is referred to as an end-of-paper citation.
It's important to cite sources you used in your research for several reasons:
To show your reader you've done proper research by listing sources you used to get your information
To be a responsible scholar by giving credit to other researchers and acknowledging their ideas
To avoid plagiarism by quoting words and ideas used by other authors
To allow your reader to track down the sources you used by citing them accurately in your paper by way of footnotes, a bibliography or reference list.