Just to be safe, you'd better take an umbrella.

Literal

Caution [for-に] umbrella [object-を] take-go [better-方がいい].

用心に (literally 'for caution') is an adverbial 'just in case / to be safe.' 持っていく ('take along') is 持つ + いく. ~た方がいい ('you'd better / it's better to') is the standard advice-giving pattern, built on the past form.