Almost every web server, network devices advertise their product version which may sometimes help attacker if a particular version is known to be vulnerable.
Having Server or X-Powered-By banner in HTTP response header is also considered as information leakage vulnerability and often asked to fix by a security scanner.
To mitigate this, you got to hide or change the version details in web server configuration. If using Apache HTTP then this guide will help you.
Once done with masking, use Server Signature tool to verify.