When you use information from another person's work, you are expected to include a citation. In most citation styles, the in-text citation goes in the main part of your paper, right next to the idea or quote. It might be a part of a sentence, an aside in parentheses, a number, or another type of reference, depending on what style you are using.
Then, there will be a corresponding full citation in your works cited, bibliography, or references page, or in a footnote or endnote. The in-text citation has just enough information for your reader to find the full citation for the article, book, or other source you used.
A librarian can help you find examples of in-text citations in a particular style, and tutors at the Writing Centre can help you format your own paper.