Greed is a bad thing. To have greed is to have a strong wish for more than you need, or more than your fair share. A greedy person wants to have and keep lots of money, lots of food, or lots of something else... and they don't care if other people have less. Someone might be greedy for money, greedy for snacks, or greedy for the teacher's attention: they want it all, and they don't care about other people's needs.
If you're sharing a cake with your friends, one of your friends might show greed by grabbing most of the cake, not caring that the others will be hungry.