In this series, Life's Little Mysteries explains complex subjects in exactly 200 words.
Time only moves forward because a property of the universe called "entropy," roughly defined as the level of disorder, only ever increases, and so one cannot reverse a rise in entropy after it has occurred. The fact that entropy persistently increases is a matter of logic: The possible disordered arrangements of particles far outnumber the possible ordered arrangements, and so as things change, they tend to fall into disarray. Imagine a stack of papers on a desk. Because there are many more ways for the stack to be disheveled than neat, any jostling has an overwhelming probability of making the stack more disheveled than it was prior to the jostling. The energy in the universe acts like those papers. It started out bundled together at the time of the Big Bang. Gradually, it's falling apart, heading toward a state of maximum entropy known as the heat death of the universe, where no single location will contain a higher concentration of energy than another (like papers uniformly dispersed across the desk). Black holes are regions of space where entropy has already maxed out. Life works by delaying the inevitable entropy increase of its atoms, keeping itself ordered by deteriorating its surroundings.