Citations are how your readers know:
Both of these things will ensure your readers that you are a reliable source and will allow them to find the background materials to confirm this information or come to their own conclusions about what you are referencing.
It is also legally required. If you take information from sources and do not cite them, it is plagiarism, which is illegal use of someone else's intellectual property. According to the student handbook, this can result in a failing grade and students are subject to disciplinary action, which can include termination from the program. So cite your sources!
These are the pieces of information necessary for proper citations:
For more specific information, see the specific style of citation and the type of work you are citing.