Scholarly sources are:
We usually talk about scholarly articles, but there are also scholarly books called monographs. These are published by university presses and have many of the same identifying characteristics as scholarly articles.
There are certain characteristics that make it easy to recognize a scholarly article when you see one.
Scholarly articles are also called peer-reviewed articles.
Peer review is the most rigorous form of quality control that exists for scholarly information sources. An expert could write a webpage or magazine article - even one intended to be read by other experts - but it would still not be considered scholarly, because the quality control process for vetting the information before it is published is not rigorous enough.
A video by NC State University Libraries explaining the peer review process.
Some library databases contain scholarly articles, exclusively. Others contain a mix of scholarly and non-scholarly sources. If you are searching a databases that mixes scholarly and non-scholarly content, or if you are using the library discovery search, there is a way to ensure that all your search results will come from scholarly sources. You will find this feature on the Advanced Search screen and adjacent to the Search Results list.
If you are using the Discovery Search, on the far left column, select "Peer-Reviewed Journals" and then click "Apply Filters." That will eliminate results from non-scholarly publications. Most databases work similarly.
Note: Not all items published in scholarly publications are scholarly articles; some scholarly publications include items like book reviews and editorials which would not be considered scholarly.