Citing your sources properly shows that you have done your research and consulted appropriate sources for your topic. It also acknowledges that all research builds on work that has come before. You are giving credit to sources that have influenced or informed your work. If you do not do this, you are essentially stealing another person's ideas, which is called plagiarism.
Whenever you use another person's ideas (even if you put this into your own words), you must give them credit. You do this by CITING the source you used in two places: