This is a famous quote from Obama's Thanksgiving speech. The original text is as follows:

But Lincoln said in his first Thanksgiving decree that difficult times made it even more appropriate for our blessings to be -- and I quote -- "gratefully acknowledged as with one heart and one voice by the whole American people."

The link to the full text of Obama's speech is as follows: http://change.gov/newsroom/entry /special_preview_of_the_president_elects_thanksgiving_address/

You can check it out. After seeing this original text, I believe you already understand what this sentence means. My translation is:

But Lincoln said in his first Thanksgiving address that hard times should make our blessings - and I quote his words - "undertaken by all the American people with one voice." Accept it gratefully with a single heart.”

For your reference.