Answer:
Years in which the sources were published.
Explanation:
When publishing a work or writing an article one is allowed to take pieces of information from already published works. In order to avoid plagiarism, those parts must be properly cited. That way, the reader can easily find and check cited texts.
There are several ways to cite someone's paper, work, article, book, online article etc.
Most of them include basic information about the author or authors, such as first and last name, the title of the work, information about the publisher, date or year of publishing and URL for web articles.
That means that years, here, suggest the year of publishing the cited articles.