Being the target of an action.
"The rain began to fall upon the garden."
In plain English: Upon means right at that exact moment when something happens.
"Upon hearing the news, she immediately called her family to share the good tidings."
Physically above and in contact with.
"The cat jumped upon the table to get a better view of the bird outside."
The word "upon" comes from Middle English and is formed by combining the words for "up" and "on." It has been used in English since Old English times to mean being located above or against something.