potential energy
Some things are loaded and ready to go even while sitting perfectly still: a drawn bow, a boulder perched on a cliff, a compressed spring. Potential energy is the energy stored in an object's position or arrangement — energy that is banked, waiting to be turned into motion. Let the bowstring go and the stored energy becomes the arrow's speed.
More precisely, potential energy is energy stored in a system because of where its parts are, held there by a conservative force (a force, like gravity or a spring's pull, whose work depends only on start and end points). What matters physically is always the change in potential energy: when a conservative force does work W, the potential energy changes by the opposite amount, ΔU = -W. Because only changes count, you are free to pick where U = 0 — the floor, the tabletop, the spring's natural length — whatever makes the problem simplest.
There are several kinds — gravitational potential energy (from height), elastic potential energy (from stretch or squeeze), electric potential energy (from charges) — and each can convert into kinetic energy and back. One honest subtlety: potential energy belongs to the whole system, not to a lone object. It is really the earth-and-ball system, or the spring-and-block system, that stores it, even though we often speak loosely of 'the ball's' potential energy.
Lift a 2 kg book from the floor to a shelf 1.5 m up. Taking the floor as U = 0, its gravitational potential energy becomes U = m g h = 2 x 9.8 x 1.5 which is about 29 J. Knock it off the shelf and that 29 J turns into kinetic energy as it falls — it hits the floor doing about 29 J of motion.
Height is banked energy: the 29 J you spend lifting the book is exactly the 29 J it gives back on the way down.
Only differences in potential energy have physical meaning; the zero point is a free choice, not a fact of nature. So a negative potential energy is not alarming — it just means the object sits below your chosen reference level.