The workspace directory is where Jenkins builds your project.
It contains the source code.
Jenkins checks it out in addition to any files generated by the build itself.
The workspace directory can also be called the root directory.
Wherever your job is triggered (whether it's on the slave or master) then that's where the workspace directory will be located.
So for example if you trigger a job in a development server the workspace or root directory will be located on that development server.
While adding the slave you will add the workspace directory in the configuration on the Jenkins web console.
Location of the workspace directory for the Master server: /var/lib/jenkins/workspace
Location of the workspace directory for slave servers:
/root/workspace
If your user was not a root user as above then the location would have been:
/home/"user"/workspace
where "user" is the name of your user.
The root user get its own directory whereas any other user is located under the /home directory.
