When something happens prematurely, it occurs earlier than expected or intended. In other words, it happens too soon, like when you show up to a party an hour early, or when flowers bloom before winter is over.
The word prematurely comes from the Latin roots prae, meaning "before," and maturus, meaning "ripe." It describes anything that happens ahead of schedule, or before its proper time. A baby who is born prematurely arrives weeks earlier than expected, before it's fully developed. A flower bud that emerges prematurely may get frozen when frosty weather roars back in late winter or early spring. A decision made prematurely, before all the facts are known, could lead to unexpected consequences.