Proofreading is important, because you have to make sure your information is accurate, as well as also provide citations or in text citations if you've quoted or taken a piece of another contributors or author's work.
Plagiarism is copying & stealing someone else's written work without giving credit, in an essay or any form of writing or conducting research, you should avoid it mainly because you'll fail. And you can even get kicked out or terminated from a college or university
Hope this helps :)
