To elaborate slightly, note that "reliable" is sort of Wikipedia jargon. When it applies to a news organization, it means that statements of fact are likely to be correct... or at least, not intentionally incorrect, because errors do happen. So a source can be reliable and biased at the same time, which means that if it says a thing happened you can largely trust that it really did happen... but any interpretation of that might be slanted, and so shouldn't be trusted.

The New York Post isn't "reliable" because it's a tabloid that doesn't care overmuch about fact-checking what it publishes (and, worse, has a history of just making stuff up sometimes). So the Wikipedia position is that you can't trust a citation to the NY Post without finding something else to corroborate it -- at which point you might as well just cite the corroborating reliable source instead.

Whereas Mother Jones will absolutely mostly publish articles which say good things about progressives and bad things about conservatives, but those things will all be true. Their bias comes in the form of selectively presenting these things -- they're unlikely to bother posting a "Ted Cruz just did a good thing" article -- and in their color commentary / opinion pieces, not in the form of just making things up.

See: https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources