When people are placed into a group and these people have to deal with some situation, the group as a whole typically has some overriding attitude toward the situation. Over time and with group discussion, the group’s attitude toward that situation may change. When it changes in such a way that the group attitude is enhanced and strengthened, then group polarization has occurred.
For example, let’s say a group of Republicans gather to discuss welfare reform and some new policy proposed by a democratic politician. The welfare policy calls for more money to be taken from private sector businesses and given to welfare centers. In the beginning of the discussion, the group as a whole may be somewhat against the welfare reform policy (thus having an initial group attitude). After discussing the policy, the group indicates that they are now more against the policy than ever. What has happened is that the initial attitude has been bolstered and the group is more polarized against the policy.