Squanto, also known as Tisquantum, was born sometime at the end of the sixteenth century. He was a Native American of the Patuxet community, which resided in Cape Cod Bay in coastal New England.
He was kidnapped by Thomas Hunt, the lieutenant of the famous Captain John Smith, and taken to Europe. We do not know much about his journey across the Atlantic, but we do know that his captors took him to Malaga, a Spanish port on the Mediterranean coast, to sell him into slavery. He was rescued and sheltered by local Roman Catholic priests, who eventually permitted him to attempt to return home. He went to London, befriended a shipbuilder, learned English, and eventually secured passage to North America on a fishing vessel. By the time he returned to Cape Cod Bay, approximately five years after his capture, his native community of Patuxet had vanished and was built over by British colonists (the Pilgrims). It is likely that his community and neighboring communities had been wiped out by an epidemic (likely viral hepatitis) brought by French sailors which ravaged coastal New England from 1616 to 1619.
Popularly, he is most famous for teaching the colonists how to sustain themselves by teaching them maize-planting and fertilization techniques. This is partially why he is often invoked in the context of Thanksgiving. However, he was arguably equally if not more influential as a guide and an interpreter. He was famous for negotiating treaties between numerous regional Native American tribes (the Wampanoag confederation) and the British colonists (the Pilgrims) of the Plymouth colony in Massachusetts. When he helped negotiate these treaties, he was actually a sort of captive of the Wampanoag confederation. He had shown up at their doorstep after his return to North America, and he was taken in but watched closely and not trusted. One of the leaders of the Wampanoag, Massasoit, worried that Tisquantum had only his own interests in mind and could potentially side with and help the colonists. However, because he spoke fluent English and had resided in Britain, he was considered too useful of an interpreter and a guide. Ultimately, Massasoit was right not to trust Tisquantum, because he tried to foment distrust in him among his people and among the colonists. Because of this betrayal, Tisquantum stayed with the colonists in Plymouth and rarely left the colony without an escort. He likely died around 1622 on a trip with the colonists to southeast Cape Cod to negotiate another pact.
For further reading, I would recommend Charles C. Mann’s 1491: New Revelations of the Americans Before Columbus, particularly Chapter 2, “Why Billington Survived.” I would also recommend the eNotes study guide for William Bradford’s History of Plymouth Plantation.