In Git & GitHub merge concept comes when we deal more than one branch.
Now what is branch??
Say,You have a file in your Machine . Now you want to share the same file to another person. So, you can do this by SSH,Gmail or any other way. But if you want to work together parallel in the same file, then the concept branch comes into place. Here you created another branch. In this both both of you work in the same file and after completion of the work, you merge the file.
Hope this will clear your doubt