Many academic journals incorporate a "peer review" or "referee" process in which other experts in the subject area examine journal articles before they are accepted for publication. A journal’s review board can send a paper back to the author with suggestions for improvement, which the author must make if the article is to be published.
Peer review makes it much more likely that the research described in a journal's articles is sound and of high quality. Note that "scholarly” and “peer-reviewed” are related but not the same: not all scholarly sources are necessarily peer-reviewed, but most peer-reviewed articles would be considered scholarly.
Some research databases (e.g., JSTOR or Project Muse) cover only scholarly or peer-reviewed journals, but most include a mix of scholarly and other types of articles (popular, industry, news).
Those databases that have wider coverage typically offer the option for you to narrow your search to scholarly, academic, or peer-reviewed (or refereed) journals. Look under "limit to" or "narrow results" in the basic or advanced search screens.
If you need to double-check whether a journal is peer-reviewed or refereed, you can try locating the journal's homepage on the web, or search Ulrich's.