A source can be considered unreliable if it is in a biased perspective or if the perspective of the source speaking is skewed in some way. It can also be unreliable if it doesn't come directly from the main source (as in, it's being told over and over again and you're just hearing it from someone else and not from the main source.) Reliable sources are usually things like interviews or journals while unreliable sources would be news articles and blogs.
