This is known as a damage modifier trait. The object with the Hydro +3 trait is weak against Hydro (water) attacks.
Whenever it is attacked by a Hydro attack, such as the Geyser Attack spell, you add 3 attack dice to the roll, and you also add 3 to the roll of the effect die (meaning there is a better chance to cause special effects and conditions like Daze).
See the "Damage Modifiers" sidebar in the rules on page 30.
