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in one line "its a cloud based code repositories's(git) web front end and used to collaborate code among team whichmay or may not be geographically co located". If you have used SVN , its smarter cousin of SVN.
Code repository, in general:
Lets say you have a team of programmers. They will need a way to share code and work as a team. Getting together to hand things back and forth is a problem. Manually updating each other would also cause a lot of extra work. So having a system that automates the process and tracks all the changes is useful. So code repositories were made for that purpose.
Over time, several types of public and prvate repositories have come into bieng. Git is a popular program for making these. GitHub is a public repository that is popular with OpenSource software that is based on Git.