(just) to be on the safe side idiom
谨慎起见；以防万一
being especially careful in order to avoid something unpleasant
I'm sure it won't rain, but I'll take an umbrella (just) to be on the safe side (= to be ready if it does rain). 我知道不会下雨，但是为了以防万一，我还是要带把伞。