Your instructors may require the use of scholarly journals, also called "academic," "refereed," or "peer-reviewed" journals. As previous noted, scholarly/peer reviewed journal articles are highly regarded sources because they are reviewed by other subject experts before being published.
Library Search and many article databases have an option to limit search results to scholarly journals, which Google Scholar cannot do.
Library Search can also limit results by publication date and to those materials "available in library" physically or online, but use these limiters thoughtfully! Here's why: