A path is a location to a folder or file in a file system of a Operating System, then is a combination of characters and "/".
An absolute path is defined as specifying the location of a file or directory from the root directory (/).In other words we can say absolute path is a complete path from start of actual filesystem from / directory.

To write an absolute path-name:

Start at the root directory ( / ) and work down.
Write a slash ( / ) after every directory name (last one is optional).
If for example, we the commands "cat group1.txt", it'll work only and only if the "group1.txt" exist in the current directory.
If doesn't works, it's not a problem, you just have to know where the file is actually stored.
Now, we suppose that you know where your file is saved so you can rewrite the command. "cat /home/a1/group1.txt".
As you can see from the last commands, the path started from "/" which is the root directory for every Unix machines.
Here are some other examples of absolute paths:
  • /home/user/Document/group1.txt
  • /root/data/dev.zip
  • /var/log/messages