In Sealed format, what does 'Limited sideboard is rest of your cardpool' mean and how is a sideboard used?

Ruling: In Sealed format, your sideboard consists of all the cards you opened but did not include in your main deck. Between games of a match, you can exchange cards between your main deck and sideboard.

Sequence:

• Open your card pool for Sealed
• Build your main deck from those cards
• The remaining cards from your pool become your sideboard
• Between games in a match, swap cards between main deck and sideboard as desired