A peer-reviewed journal article is one which has been through a rigorous process of scrutiny and criticism before finally being published. An author submits their article to the journal, and several academic experts in that field will read, criticise and comment on it, returning it to the author to make improvements and clarifications before re-submitting it. This process is intended to ensure that when an article is published, the reader (that's you) can be sure that it is based on scholarly, ethical and reliable research and information.
The diagram below explains the peer-review process:
It is a very good idea to use peer-reviewed journal articles in your coursework; they will be high-quality, scholarly material which is what your tutors will expect to see you using.
Many of the journals held by the Library are peer-reviewed. Some of the online databases also allow you to filter your search to show peer-reviewed journals only.