Why is Jonas confused when the Receiver uses phrases such as "the whole world" or "generations before" him?
Jonas tells the Giver that these phrases confuse him because he does not know that there is anything other than "only us" and "only now." We see this in Chapter 10 when Jonas goes and meets the Giver for the first time.
I think that the purpose of this passage is to show us how closed the minds of the people in the community are. They cannot imagine that other kinds of people exist and that there has been a past that was any different from what they have now. The society has imposed Sameness on them so much that they think that everyone in the world has been like this at all times.
Jonas and his family, as well as most of the others in Lois Lowry's novel, The Giver, have virtually no access to books or other forms of historical reference. They know little of what has happened before their own lives or about the outside world. They do not know who their birth parents are, so the term "generations before them" has little significance. Their "whole world" is simply what is contained around them; beyond their homes is simply referred to as Elsewhere. Only The Receiver is privy to such information, so it is not surprising that Jonas is confused with these terms.