Bidding Game -what is it?

 Bidding to 'game' means you have bid to the level where you are entitled to extra bonus points just for bidding up to that level. If your contract stays below the level of game it is then called part game and your bonus is much smaller. To obtain this bonus you must bid and make your contract, if you don't, points are awarded to your opposition but on a different scale. The partnership vulnerability affects only the 'game' bonus points not the 'part game' contracts and many more points are allocated for a vulnerable game contract.

'Game' or 'part game' bonuses are allocated at the end of each round as follows:

• any part game contract successfully bid and played, scores a bonus of 50 points on top of points for the tricks made
• any 'game' contract successfully bid and played, scores a game bonus of 300 if not vulnerable and 500 if vulnerable on top of the points awarded for the tricks played and won.
• if you fail to make your contract the opposition gets points allocated, you and partner get none (when playing against the computer it is recorded as a minus score eg. -200)

If a partnership have enough points in their combined hands, they should bid always try to bid up to the "Game" level.
You need at least 25 HCP between you and your partner to bid game in NoTrump (i.e. 3NT), 26  TP  for game in 4♠ or 4♥, and 27 TP  to bid game in 5♦ or 5♣. These point requirements are approximate, subject to partnership agreements, and don't guarantee making the game contract. 
If responder has 13 points or more, then it is right to bid game, because Opener already has shown an opening hand and responder has extra points: 13TP  (opener) + 13TP  (responder) = 26 TP

The Number of Points and Tricks contracted to achieve Game"  · Bidding 3NT requires 25 HCP and winning 9 tricks · Bidding 4♠, 4♥ requires 26 TP and winning 10 tricks · Bidding 5♦, 5♣ requires 27 TP and winning 11 tricks