It means that we may agree on many things, there are some things that we don't agree on. How many things we agree on can be different, depending on the situation. Usually it means that we agree on most things, but not everything. "We agree on some things." - There are things that we agree on, but it is implied that there are definitely things that we don't agree on. "We don't agree on much" - We disagree on most things. "We don't agree on anything" - There is nothing we agree on.