If you use someone else's idea in your writing, you must credit that person for his/her idea. This is known as referencing. Referencing is very important in academic writing. It helps build up your own argument by showing that you have support from other authors. This allows you to build on the ideas of others without reinventing the wheel.
If you don't reference, then you are effectively stealing someone's idea. This is a very serious offence in the academic world.
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.