Upon is a more formal way of saying “on,” used mainly in writing or when speaking formally. When Goldilocks entered the cottage, she saw three bowls of porridge upon the table. A knight's armor rests upon his shoulders.

You'll find this word frequently in books, especially older ones or fairy tales. “Once upon a time” begins countless stories, though you couldn't naturally say “once on a time” instead. People write upon when they want their language to sound more elevated or traditional.

The word also appears in some common phrases where switching to “on” would sound odd or change the meaning slightly. When something happens upon arrival, it happens right when you arrive. If you stumble upon something, you discover it by chance, like finding a rare coin while digging in your backyard. These phrases have stuck around even in everyday speech.

In most writing, you can usually replace upon with “on” and sound perfectly natural. Upon is more common in formal or literary writing, or in set phrases where it just sounds right.