When your teachers talk about scholarly literature, they often mean that your sources should be of the kind that has gone through the process of reviewing by independent subject matter experts. This is known as peer review, and is specifically essential to scholarly articles. In many databases you can choose to filter your search to include only articles from journals which use peer review. Here is an example from Primo of what it might look like:
If the search filter for peer review is lacking in the database, you can use the search service ULRICHSWEB (link) to check if a journal uses peer review. Enter the journal name in the search box and then look it up in the results list, check for the symbol 
signifying "Refereed", which is equivalent to peer review, as in the example below:
Scholarly peer-reviewed journal issues may also contain other types of articles (debate, reviews and so on). These types of article are not scholarly reviewed. To be quite certain that an article is scholarly you can check a number of its features like the following examples: its structure, the number of pages, a reasonably long reference list.
To determine if an article is scholarly, one can also look at how it is structured. Usually the arrangement of the structure's parts follows a scheme called IMRAD (after the initial letters of the headings listed below). The structure is very similar to the one of a student essay:
This structure is more or less standard for scholarly articles and authors often use these headings to name the parts of the article. Note, however, that this structure only applies if the study is empirical, that is, the researchers collected their own study material through, for example, interviews, surveys, experiments or the like. More theoretical articles often have a freer layout.
Another type of scholarly literature is the doctoral dissertation. The doctoral dissertation is also subject to expert review, though in a different way from scholarly articles. If your teacher has asked you to find scholarly articles, the teacher will also often accept doctoral dissertations (provided you have time to read them!) as an alternative to the article. In some databases you can choose to limit your search so that you only search for doctoral dissertations. Here's how it looks in Primo:
Sometimes even books are expertly reviewed before being published (though this is rarely the case for books written in Swedish, so this applies mainly to books in English and other languages). Unfortunately, there is no easy way to see if a book has been reviewed. For books that consist of chapters written by different authors, there is always an editor who reviews the contents of the book, but that form of review is not considered to be comparable to peer review.