When you use someone's idea without acknowledging them, you are committing plagiarism. This happens when you use someone else's words exactly as they have without referencing the quote correctly, or if you paraphrase an idea without giving the original author credit for it.
Plagiarism is serious. If you plagiarise you could get zero for your essay, fail the course, go to university court or get kicked out of varsity effectively ruining your academic career. You can avoid plagiarism by referencing correctly.
The following document gives more information on plagiarism. It explains UCT's plagiarism policy, and fully explains how to avoid it.