Peer review is a process whereby experts in your field critique your research. The process is carried out independently and the aim is to ensure the quality of the research and its publication suitability. The process also is important as a useful source of feedback for your self and your fellow researchers. The reviewers critiques point out potential weaknesses in your paper and suggest ways in which improvements can be made. Some people find this process intimidating, whilst others see it as a really positive experience. The key is not to be down hearted if you submit a paper and it comes back with lots of comments for developing.
The other side of peer reviewing is that it gives the reader of articles published confidence that the quality of the paper is high and the arguments contained within are robust.
Some databases eg CINAHL have a limit filter allowing searching for peer reviewed papers - but please note that by using tools like this you may be filtering out relevant results.