Peer reviewed resources can be considered a golden standard for credibility, as the work should have been reviewed by experts from the field and validated by them. It should follow industry standards, use the best practices and methodology, be well read, well referenced and fit within the current accepted truth about the field.
Reproduced research often isn’t published alongside a paper.
Less credible resources aren’t necessarily useless, although usually a poor choice for referencing information. Less formal sources can be used to get a feel for the field of study and what’s been happening recently, or to reference specific information used in the research yourself, such as a section of code.