The claim in the Declaration of Independence is that the thirteen colonies are in their right to break free from the English ruling and establish its own government. And so Jefferson proceeds to explains the reason for this. This is established in the first paragraph.
A concession is a recognition of some of the arguments opposite to the claim. In the proclaim, Jefferson recognizes that is true that "Prudence, indeed, will dictate that governments long established should not be changed for light and transient causes; and accordingly all experience hath shown that mankind are more disposed to suffer, while evils are sufferable than to right themselves by abolishing the forms to which they are accustomed".
He recognizes this as true, but his argument is the opposite. So he presents the evidence of the tyrannical actions the English government has had against the colonies, which entitles them to break free and form their own government.
