It's the masculine form of the Italian adjective "serio" (meaning "serious"), and is used to describe something or someone that is grave, important, or significant.
Here are some examples of how you might use "serio" in English:
* "He has a serious problem with his health."
* "The situation is becoming serious."
* "This is a serious matter."
* "He's a serious student who works hard."
The feminine form of "serio" in Italian is "seria."