to be on the safe side 谨慎起见；以防万一

(just) to be on the safe side的意思是谨慎起见；以防万一，直接用在句末就可以了。

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。