You have the option within a game to designate certain questions as Bonus Questions. Bonus Questions (unless you select a different Question Play Rule) can only be answered by the team that picks the Bonus Question. Bonus Questions also allow teams to "wager" a certain point value, so they allow teams to make up large point deficits (or to extend them).
To set Bonus Questions in your game, simply click once on any of the questions indicated on the board. Instead of its point value, the word "Bonus" will appear in the question box:
To de-select a Bonus Question and convert it back to a regular Point Value question, click on the cell again.