Siddhartha Gautama had ben brought up in a privileged royal family and had been kept safe from all of the bad things in the world.
One day he decided to go outside and see the world and during this he saw four things the first one being an old man. This affected Siddhartha as he did not know of ageing.
The next sight he saw was death and this was something that he had never seen before so it had affected his mind quite strongly.
Another sight he saw was a sick man this caused Siddhartha to feel worried as he did not want to become sick.
The last sight he saw was an ascetic. This is a person who gives up everything and lives the most simple life. This made Siddhartha think that he should be living like that and not care about materialistic items.