Thereon means on or onto the thing just mentioned. It's a formal way of saying “on that” or “on it,” and you'll mostly see it in old books, legal documents, or official writing.

In a fairy tale, you might read: “The knight found a stone tablet with ancient runes carved thereon.” This means the runes were carved on the tablet. A contract might state: “The property and all buildings thereon shall be sold,” meaning all buildings on that property.

While it sounds old-fashioned, it's still used in formal contexts where precision matters. In everyday conversation, people would simply say “on it” or “on that” instead. You might encounter thereon in historical novels, where characters speak more formally, or in legal documents, where traditional language persists.

Related words include therein (in that), thereof (of that), and therewith (with that). These formal compound words were once common but now appear mainly in specific contexts. If you're writing a story set in medieval times, thereon fits perfectly. For regular conversation or modern writing, stick with simpler alternatives.