Peer-reviewed articles (sometimes called refereed articles or generically called "scholarly" articles) are a very specific kind of source.
If an article is peer-reviewed, other experts in that field have critically reviewed the article for content (looking at the research process, the conclusions vs the evidence, and so on). They may make suggestions for revision before the article is allowed to be published.
Psst -- what's a database?
In its most basic sense, a database is just a selection of information designed for you to search and retrieve stuff from it. Amazon is a database you're probably familiar with: it's limited (only contains things you can buy through them) and retrievable (you can search and filter your results to find what you're looking for).
The library databases contain reputable, reliable sources of information to support researchers like you! This means everything from digital encyclopedias (like Credo), ebooks, scholarly journal articles, magazine and newspaper articles, streaming videos, statistics, and more.
The Three Rs
Is your source
Recent? Reliable? Relevant?
Is this source up-to-date? Is it about my topic, and does it go into enough depth? Does it come from an authoritative source? Is the information accurate (and are there citations given to back it up)? And why was this information written in the first place?