Why does Jonas have to start taking pills? What happens to children who are released?
Jonas begins to take pills because he begins experiencing "stirrings," which is really another word for sexual desire. In the community people do not talk about sexuality and it is physically repressed via the pills. It appears that most citizens (perhaps there are exceptions we are not made aware of, like for Birth Mothers or the Giver himself) have to take these pills in order to repress their sexual desires, which arise around puberty.
Children who are released from the community are actually given a lethal injection and killed. "Released" is another word for "killed." However, outside of the people who actually perform the releases, like the nurturers for example, the majority of citizens do not seem to realize that this is what "release" means. We see this in Ch. 4 when Jonas is volunteering at the House of the Old and the elderly woman, Larissa, says she does not know what release entails, only that the man who was released, Roberto, looked happy just before entering the release room. This implies that the actual details of release, for all ages, are kept secret from most people.
Jonas has to start taking pills because he is experiencing "stirrings." What this means is that he is becoming sexually mature and starting to have sexual thoughts and desires. No one in the community is allowed to have these because it would disrupt the Sameness that is imposed on all members of the community.
Children who are released (anyone who is released, actually) are killed. Release is just a euphemism for killing. We see a video of Jonas's dad killing an infant with a lethal injection and dumping it down a garbage chute. This is what really makes Jonas decide to leave the community.