The "horizon" is the line where the sky and the earth seem to meet each other when you are looking at the ocean or a flat area of ground.
When you look at something large that's very far away, it might look like it is touching this line. In that case, you can say that it is "on the horizon":
I see something on the horizon.
People also use the phrase "on the horizon" in a figurative way. They use it to talk about something that's going to happen in the near future. For example:
We have some interesting new projects on the horizon.