When “scholarly sources” or “peer-reviewed articles” are expected for an assignment, this has a specific meaning. It means a certain class of journals or books, generally published through academic presses, that employ certain methodologies to ensure that publications are vetted by the academic community. Such publications have a level of prestige and reliability in academic discourse greater than that of journalism, raw data sets, or other research data. Scholarly articles are not necessarily better than other types of publication, but they do play a distinct and important role in the generation and circulation of knowledge.
It is not always immediately obvious what is and isn't a peer-reviewed, scholarly source, but students should make an effort to learn how to determine this, as it will be useful for you throughout your academic career. Here are some good guides:
A key takeaway: There is no one universal way to determine whether something is peer reviewed, but with some practice, it usually isn't hard.
Peer-reviewed sources can come up in normal Web searches, but there are also dedicated search tools that can help facilitate finding scholarly publications:
These don't guarantee that all search results will be peer reviewed, but most will be. Always be sure to check yourself (such as by reading the “about” page of a journal) if there is any uncertainty.