The sentence is a paradox, Guardas a chuva, páras a água means that you keep the rain and stop the water. Plays with the Spanish word for umbrella, paraguas, and Portuguese word guarda-chuva.

Guarda it has two kinds of meaning in Portuguese, protect from, and keep something with you.

This drawing tries to catch the way that people hold their umbrella. On some way the umbrella is an instrument that keeps you safe and protect you from the water, but this relation between hands and umbrellas has always water in the middle, and the relation gets personal with hands. Although the umbrella keeps your body safe from water, your hands are the way to get you into the water.