The word "source" as used on Wikipedia has three related meanings: the piece of work itself (the article, book), the creator of the work (the writer, journalist), and the publisher of the work (for example The New York Times, Cambridge University Press, etc.). All three can affect reliability. Reliable sources may be published materials with a reliable publication process, authors who are regarded as authoritative in relation to the subject, or both.
The reliability of a source depends on context. Each source must be carefully weighed to judge whether it is reliable for the statement being made and is the best such source for that context. In general, the more people engaged in checking facts, analyzing legal issues, and scrutinizing the writing, the more reliable the publication. Sources should directly support the information as it is presented in an article, and should be appropriate to the claims made. If a topic has no reliable sources, Wikipedia should not have an article on it.