Plagiarism is claiming another person's work as your own.
That sounds like something a normal, honest person wouldn't do, but you can "accidentally" plagiarise if you aren't paying attention.
The most obvious forms of plagiarism are:
Now the first two are obviously cheating, but the last one can happen by accident if you don't pay attention to your sources of information.
Any information that you did not invent must come from somewhere.
When you don't acknowledge the work other people have done, you are saying "This is my original work" and, if that isn't true, then you've plagiarised.
Plagiarism is very simple to avoid. All you have to do is acknowledge where your information came from.
Step One: Whenever you read something, take notes about:
Step Two: When you are thinking about your essay:
Step Three: When you are writing your essay: