To be credible means to be trustworthy and reliable.
To increase credibility individuals and companies often get themselves legally registered or get verified from a recognized government or private
institute through certifications.
This shows their clients that their skills and services will be good.

Example:
For example, doctors usually put M.B.B.S and M.D on their prescription. This shows that the doctor is qualified. Since the doctor also holds an M.D this means they are more experienced and credible than a regular M.B.B.S doctor.