The Joke:
What do french fries do when they haven't seen each other in a while?
Ketchup..
Literal meaning (food)
French fries are usually eaten with Ketchup, the tomato sauce.
Hidden meaning (the real joke)
The phrase “catch up” means to talk and share what has happened after not seeing someone for a long time.
Example:
We haven’t seen each other in years. Let’s catch up!
The joke mixes the two meanings:
French fries → ketchup (the sauce)
Friends who haven’t seen each other → catch up (talk about life)