Plagiarism is when someone copies another person's work and takes credit for it themselves. It can be a serious problem, and is definitely something to keep in mind when you're writing your essay.
Avoiding plagiarism doesn't mean you can't use other people's work to support your ideas. It just means that you need to record any quotes or ideas that aren't your own, and reference them in the body of your essay, and in your bibliography.
Your school will have its own policy on what constitutes plagiarism – it's a good idea to find out what this is, because when it comes to plagiarism, 'I didn't know' isn't an excuse.