If you don't specify anything on startup your workspace will have the name workspace and will be located at your current directory (not necessarily the directory of the application). Knowing the location of the workspace folder is useful for a variety of reasons including viewing the .log file where you can see errors that may have been thrown by the RPG Next Gen application. You can find the .log file at workspace/.metadata/.log.
If you want your workspace somewhere else to be you can use the data option like this:
rpgng -data /home/mihael/.rpgng-workspace
This will give the workspace the name .rpgng-workspace (on linux the folder will be hidden because of the leading dot) and puts it in the home directory of the user mihael.
By default the workspace directory in Linux will most likely be at the following location:
/home/<username>/workspace