Value is an object, which each list item has. When you create list items, you can assign a value object to them, and that object will stay with the list item (but not be displayed on the page).

The ideal use for this, is when you have a list of items that corrospond to ID numbers in a database. You would set the text property to the friendly description for the item, and the value to the ID number. When you go to interact with your database, you don't need to parse what ID number has been selected; because it's just in it's Value.