There are many secrets to be found around the Retroverse, secrets
which will only be revealed to the diligent. To aid in giving an
explorer a sense of how far he has progressed, he can see the percentage
of the Retroverse he has explored in his 'score'. It is also rumoured that
if the explorer takes the time explore with open eyes, he will earn some
experience when visiting new locations other than the outerworlds for each
of the six worlds.

Exploration gives experience. The greater your exploration 
percentage, the greater the experience you receive.
Note: this means you cannot be in brief mode, nor during a running
battle.

See also: HELP EXPLORE