Lois Lowry's The Giver has more than one theme. I'm not sure which theme you are asking about. I'll take a stab at it though with one of the more common and prevalent themes.
The theme of memory is important to the story. Memories can be good or bad. The concept behind The Giver, though, is that all bad and painful memories have been eliminated from society. People no longer remember past events that were hurtful. The only person that is capable of remembering is the Receiver. The elders recognize that there might be a time when those past memories are useful for advice, but not for feeling. This emotionally lopsided society is seen as a good thing. I'm not going to lie, it even sounds kind of nice to me at times.
The theme changes, though, as Jonas learns that he can better understand pure happiness when he is able to compare it to pain and suffering. The theme of memory shifts to showing the reader (and Jonas) that all memories are important. Both good memories and bad memories. That concept becomes so important to Jonas and the Giver that they devise a plan to release all of the memories to people at once.