Something that's historic was important in its day and continues to be studied and remembered, like the historic integration of segregated schools and the historic March on Washington in 1963.
When you describe something that's historic, it's either the first or the only of its kind. Historic moments are so important or momentous that they'll show up in history textbooks. A building can be historic, and so can a speech or a battle or a disaster. The root word is the Greek historia, "a learning by inquiry or history," although historic specifically means "important in history."