A substance that decreases hand increases ph when it dissociates in water is called a base
A base is a substance containing (OH)- group (hydroxide group) in its formula. This means that when a base is dissociated in water, hydroxide group (OH)- will be released in the water which leads to the decrease in the concentration of the hydrogen and increases the pH.
On the other hand, an acid contains H+ in its formula. This means that when acid dissociates in water, hydrogen is releases, thus, increasing hydrogen concentration and decreasing pH value.