As you write your paper and use the ideas and words of others, you will need to provide a citation to tell your readers where they can find the original source of these words and ideas.
This tab of the guide will tell you when you need to cite and how to cite. It's a basic introduction to citing. If you already know about citing and just want some tips to simplify your life, move on to the "Citation: online tools" tab.
There are many ways to learn the ins-and-outs of APA style. Here are some.