"Reticulating Splines" was one of many possible loading screen phrases for games like The Sims and Sim City.

Instead of realistic ones like "Loading assets" and "Reading file", they would give humorous nonsensical ones like "Reticulating Splines".

SimCity 2000 showed a message "Reticulating splines" when it generated the landscape that didn't mean anything in particular.

It became a bit of a meme in the years since.