An implosion is the abrupt, violent collapse of something large. When an old building needs to be removed to make way for new structures, implosion is often used as a controlled way to destroy it.
A real scientific implosion happens when the pressure inside and outside of an object vary so much that the object is crushed, or collapses in on itself. This can happen, for example, to a submarine under water or to a collapsing star in the galaxy. The implosion of a building involves careful small explosions that make the middle fall as the sides collapse inwardly.