It’s all explained on the Olympic website.
http://www.london2012.com/equestrian/event/jumping-team/competition-format/
[I]Competition format
There are a total of five rounds in the Jumping competition, with all five playing a part in the Individual event. Two of these rounds are used to determine the placings in the Team event.
First round: First Individual competition
All riders compete in the first qualifier. The best 60 Individual riders (including those tied for 60th place) and all Team riders ranked lower than 60 progress to the second round.
Second round: first round of Team competition and second Individual qualifying competition
The best 45 individual riders (including those tied for 45th place), based on combined scores from both rounds, and the best eight teams (including those teams tied for eighth place), based on scores from this round only, progress to the third round.
Third round: second round of Team competition and third Individual qualifying competition
The final placings for the Team event are determined by the sum of the scores in the first and second round of the Team competition.
After the third round, the best 35 riders (including those tied for 35th place), based on the combined scores, with a maximum of three from each country, progress to the fourth round.
Fourth round: Individual final round A
The best 20 riders (including those tied for 20th place), based on scores from this round only, progress to the fifth round.
Fifth round: Individual final round B
The final placings for the Individual event are determined by the sum of the scores in the Individual final rounds A and B.
If two or more rider/horse combinations are tied for medal places after the fifth and final round of the Individual event, there is a jump-off to break the tie.[/I]