Writing can generally be classified into two categories, popular or scholarly. Some indications that something is scholarly: authors are clearly indicated, citations and/or a bibliography are included, is published by academic presses or scholarly/professional organizations, and conclusions are based on the evidence provided.
While you will usually need scholarly materials for most of your research, sometimes popular sources can offer good background information. For certain topics popular sources are more abundant than scholarly ones. For example, while there are peer-reviewed journals about popular music, if you need information about a recent concert or band, a popular source like Billboard or Rolling Stone may be what you need.
Scholarly resources may also be peer-reviewed, meaning other scholars have looked at it and vetted it as good research before it's published. Popular writing may be edited by someone for style and clarity, but is usually not reviewed rigorously for content.
The following guides from the Undergraduate Library can help you learn to determine when something is scholarly or popular.