“Better safe than sorry”, or “prevention is better than cure” in English. It is the favourite expression of the most cautious people.
This Spanish proverb means that it is much better to prevent something bad from happening rather than having to fix it once happened. That is, it is preferable to take the necessary measures before, instead of having to fix it later.
You will also find the “más vale prevenir que lamentar” variant, although it means exactly the same.