In this chapter, Jonas is thinking about how much the memories that the Giver has given him have changed his life. He is thinking about how those memories make him different from his family.
What he is saying in this chapter is that their feelings are not real. They think they know what it means to be angry or sad, but they really do not. It's as if they know the definitions, but not the actual way it feels to be sad or angry.
Jonas has actually felt the real things now and he realizes that they can't be made to go away quickly -- his feelings are much more intense and real than the versions that his family "feels."