The phrase suggests actions are taken to prevent potential problems, even if they seem unlikely. He decided to save the document frequently to be on the safe side.
Pro Tip 2/3
Not Just Fear
Using 'on the safe side' does not mean one is fearful, but rather careful and thoughtful. She double-checked her essay to be on the safe side before submitting.
Pro Tip 3/3
Casual Expression
It's commonly used in everyday conversation rather than in formal writing or speeches. I might wear a jacket to the party, just to be on the safe side.