Here's the context, the last paragraph of the speech (available here).
People of Berlin – and people of the world – the scale of our challenge is great. The road ahead will be long. But I come before you to say that we are heirs to a struggle for freedom. We are a people of improbable hope. With an eye toward the future, with resolve in our hearts, let us remember this history, and answer our destiny, and remake the world once again.The word improbable means "not likely to be true or to happen," and "unexpected and apparently inauthentic." It's synonyms include: doubtful, dubious, debatable, ridiculous, absurd and preposterous.
What, one can reasonably ask, what is untrue, unlikely, absurd or ridiculous about our hope? Did Obama say that we're unlikely to have hope? Or is just another nice sounding, but empty phrase that has characterized Obama's pontifications?