The Target Volume has two meanings depending on the Gain mode you chose.
If you are in Radio mode, then the Target Volume is the volume level that you would all files to be corrected to.
If you are in Album mode, then the Target Volume is the volume level that you would like the entire file collection to be corrected to while still keeping their volume levels relative to each other.
Doing some analyses on several of your albums will give you a feel for which values generate louder or softer sound. Experiment and see what you get.