What did the word "frightened" mean, according to Jonas?
To find this answer, you could look at the very first paragraph of the very first page of this book. It is right there.
It says that "frightened" is a deep, sick feeling that something terrible is going to happen. Frightened is the way he felt the time, about a year ago, when the airplane had flown over at a time and place that it was not supposed to be.
This is in contrast to the feeling he has now, about the upcoming Ceremony of Twelves. Now, I think, he is more nervous or apprehensive.
Initially the word frightened did not have a meaning to Jonas really. In the community all emotions were muted. However, once the Giver transfers the memory of the soldier dying on the battle field, Jonas comes to realize that frightened is a horrible feeling.
After experiencing death and the pain and sadness and terror involved in death he witness the process of the release. He become frightened for Gabriel because he knows he will be put to death if he is released. He now knows the meaning of being frightened.