What does 401/201 stands for in the DEV401 Developer & ADM201 Administrator certification? Is this just a number only or any other significance in salesforce?
"ADM201 certification" or "DEV401 certification" are commonly misused labels for the Salesforce Certified Administrator and Salesforce Certified Developer certifications (the latter has recently been replaced by Salesforce Certified Platform App Builder).
ADM201, DEV401 etc. are training courses offered by Salesforce that loosely correlate with the certifications. These courses may be taken in order to help prepare for the certifications, but they aren't required in order to take the certification.
These are just numbers which also, like high school or college classes, suggest difficulty levels as in 200 level and 400 level courses or certifications. That having been said, the DEV 401 Certification is no longer available and has been replaced by the Platform App Builder certification.