The Subscreen shows Link's current inventory, the Triforce pieces he has collected, and other items. Pressing the Start button displays the full (or "Active") Subscreen. A smaller portion of the Subscreen (the "Passive" Subscreen) is usually shown during normal gameplay. The Passive Subscreen usually shows what weapon and item Link can use with the B and A buttons, as well as his current number of Rupees, Life, and Magic.
In custom quests, the Subscreens can be customized using the Subscreen Editor.