Trustworthy, reliable.
Credible sources are generally understood to be accurate and reliable sources of information, free from unfair bias.
Who wrote the source and what authority do they have to write about this topic?
What evidence do they use to support their claims? Would you consider that evidence to be reliable and credible?
Inclination, leaning, prejudice, predisposition
A biased source is one in which the creator has a view of the issue at hand that had an effect on how they created the source. From the synonyms above, you can see that this can be to a small or large degree. Everyone has biases, and someone with a bias can still write a worthwhile source, but it is up to you to consider how much of a bias is present. Be aware of the biases inherent when an organization has a legislative agenda or is trying to sell something or is perhaps trying to appeal an audience with a set of investments and values.
Peer review is a process scholarly articles go through before they are published. Scholarly articles are sent to other experts in the field (peers) to ensure that they contain high-quality, original research important to the field. This is a measure of quality control other types of literature don't go through.
If you can't tell whether or not a journal is peer-reviewed, check Ulrichsweb.
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