The 12k bots on pandanet are good for learning how to make large groups live against a relatively good opponent, large group deaths are usually the most damaging thing to a beginner's results.
In your large-looking territory, the territory would be the eyes.
Large group death just basically mean a large group of stones, it could either be an invasion group, an attack and run group that grows, or even the used-to-be-territory-but-now-not group