In general, articles labelled "advance online publications", "early access", "epub before print", or "published online before print" describe an early edition of an article, which has been peer reviewed but not yet assigned a volume or issue number for a journal edition.
"Preprints" are articles that have not yet been peer reviewed, but are being published for the purpose of awareness and informal feedback, usually in a preprint repository such as medRXiv.
You are able to use these articles in your reference list, provided you include the author, date of the version you used, DOI, or URL and the article and journal titles. You might include a descriptive statement, depending on your referencing style. See the links below for more information.
