At the 'Main' menu go to 'Options' and then 'Credits' and press the question mark button to access the 'Password' menu'. Then tap the icons to change them and hit the 'Confirm' button when it is the correct combination.
2000 Gem Bonus (can only be used one time):
Tap (top row:)swords, armor, ring (bottom row:)gem, gem, gem
Extra Heart (only works if you have less than 5 hearts):
Tap (top row:)armor, armor, armor (bottom row:)ring, ring, ring
Unlock ALL Chapters:
Tap (top row:)armor, swords, armor (bottom row:)armor, swords, armor