An event is considered historically significant if most historians concede that the event has had important ramifications. Some events are historically significant only to a region, while others affect the entire world.
Historically significant events consist of many different types. Inventions, wars, influential speeches and books, social movements and the unique actions of inspiring individuals can all be of historic significance. For example, the invention of a flying aircraft in the 20th century revolutionized the entire world, and the 1848 Seneca Falls convention set off American first-wave feminism.
Consensus is important in determining historical significance. A historian's ancestors are of critical importance to him, but it is unlikely that his ancestors were of historical significance to the wider world. When teaching history, an instructor may single out random individuals who were part of a movement, but it is still the movement that is historically significant and not the individual.
Once an event's historical significance has been determined, it can be applied to other things. For instance, an antique cradle by itself may not be significant, but if it's the Peregrine White cradle that came to Plymouth, Ma., on the Mayflower, it is associated with a significant event and becomes a priceless cultural symbol. When historical societies and government agencies decide what buildings or areas to protect as historical landmarks, it is in part this historical association they are considering.