The Four Passing Sights (or simply Four Sights) were four things seen by Prince Siddhartha when he went outside his palace for the first time after being confined to that palace by his father. The Four Sights made him think about life more deeply and, eventually, set him on his journey towards becoming the Buddha.
The four things that Siddhartha saw were an old man, a sick man, a corpse and an ascetic. The first three of these helped Siddhartha to understand the human condition and the last helped him to understand what he should do with his life. The first three sights showed Siddhartha that all of life is pain and suffering. The fourth sight gave him an idea as to what he could do to help relieve people from this suffering.
In these ways, the Four Passing Sights set Siddhartha on his way to becoming the Buddha.