Citation management tools allow you to organize your research. In addition to saving citation information and linking to PDFs, many programs also assist with in-text citations and bibliography formatting.
In academic writing you must cite any text or idea that is not your own original work. Even if you phrase an author's idea in your own words, you must cite the original source. This is to give credit to the original author, and also so that your reader can trace your sources and references.
Proper citation usually includes in-text citations or footnotes to indicate where and when you are referencing a source, and a bibliography or reference list with more complete information so that the source can be located if necessary.
There are many helpful guides online (e.g. those in the box below) that will help you to cite your sources. There are many citation styles with their own rules for style and formatting. Have a look at the tools, guides, and online resources, or Ask A Librarian if you have any questions!