When you look out your window and note the furthest point you can see––the line where the sky meets the earth––that edge is called the horizon. Horizon can also mean the edge of something in a figurative sense.
When you are thinking about your life, it can be difficult to get very far beyond where you are. Teenagers have a hard time imagining beyond the horizons of young adulthood. People expecting a baby often put much of their focus on infancy––having a child who can sit up or even walk is beyond the horizons of their thinking.