The new words that Jonas learns are important because they represent the first time that he is learning about anything other than what the society has told him. It is the first time that he is being exposed to concepts that are outside of his experience of Sameness.
In Chapter 11, Jonas learns words for various things that don't exist in his society. He learns about snow and hills, and sunshine. These are all things that in some way have been done away with by Sameness and Climate Control. The words are important because, by learning about them, Jonas is going beyond the bounds of his society and is starting to become different from the other people in the community. This trend will culminate in him leaving (and destroying) the community.