Jonas was fascinated enough by the apple to be willing to steal it from the recreation area. When he and his friend had been playing catch with the apple, Jonas had noticed that it seemed to change in midair. At the time, he couldn't explain that change, but he was very curious about it and wanted to find out more or see if he could make it happen again, so he took the apple home.
Later in the book, when he meets The Giver, Jonas realizes that he is seeing the color red. This is what the Elders call "the seeing beyond," and in Jonas's case it manifests itself in the ability to see color. In their society, no one else has the ability to see colors at all; everything is bland and neutral. But Jonas begins to see the color red in the apple, and later in the faces of the community members and in Fiona's hair. This is an indication that he will make a good Receiver.