Plagiarism is the use of someone else's words or ideas without giving them credit. Plagiarism can be as simple as buying a paper on the internet and passing it off as your own, or as complex as using your own words but failing to cite where an idea came from.
We all get information from outside sources, especially when writing research papers. When you get ideas, phrases, or words from another source, it's okay to incorporate them into your essay, as long as you cite where you got the information.
The general rule is: when in doubt, cite it!
This short video from Southeastern University's Steelman Library gives a definition of plagiarism and five tips for avoiding it.