Here's an article about it to place it context:
The article discusses how graffiti in ancient Rome wasn't like it is today in say, New York or London. It was not something just delinquent kids do to deface things. It was something everything did both inside and outside of buildings for a variety of purposes.
The following is a link to some translations. Please note that many of these are extremely bawdy and/or sexual in nature.
This one just made me laugh because it was sublimely mundane:
"On April 19th, I made bread."