Thomas Jefferson is credited with being the main author of the Declaration of Independence.
When the Continental Congress was considering declaring independence, they appointed a five man committee to write a draft declaration. The committee, in turn, appointed Jefferson to write that draft. After Jefferson wrote his draft, the committee and later the whole Congress read it and suggested changes and deletions. However, historians still credit Jefferson with being the main author of the document. This is why the loc.gov link below says that, even after the revisions, Jefferson "retained his prominent role in writing" the Declaration.
So, Thomas Jefferson's contribution was that he was the main author of the Declaration of Independence.
Drafting the Declaration of Independence in 1776 became the defining event in Thomas Jefferson's life. Despite Jefferson's desire to return to Virginia to help write that state's constitution, the Continental Congress appointed him to the five-person committee for drafting a declaration of independence. That committee subsequently assigned him the task of producing a draft document for its consideration. Drawing on documents, such as the Virginia Declaration of Rights, state and local calls for independence, and his own draft of a Virginia constitution, Jefferson wrote a stunning statement of the colonists' right to rebel against the British government and establish their own based on the premise that all men are created equal and have the inalienable rights of life, liberty, and the pursuit of happiness.
Through the many revisions made by Jefferson, the committee, and then by Congress, Jefferson retained his prominent role in writing the defining document of the American Revolution and, indeed, of the United States. Jefferson was critical of changes to the document, particularly the removal of a long paragraph that attributed responsibility of the slave trade to British King George III. Jefferson was justly proud of his role in writing the Declaration of Independence and skillfully defended his authorship of this hallowed document.