When we describe a source as "credible," we’re basically saying that the information is high quality and trustworthy. Essentially, that we can believe what the source is telling us.
When you use high-quality sources to back up your points, you demonstrate your own credibility as a writer, thereby contributing to the overall effectiveness of your argument. The best quality research builds on other high quality research. This is true of both your own work and the work of professional researchers.
(This explanation from North Carolina State University Libraries is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License)
You should always use a credible source.
Scholarly sources are credible; but credible sources are not necessarily scholarly. For example: an article from USA Today newspaper or Time magazine is credible, but it is not scholarly.