Humidity can reduce static electricity. Water conducts electricity, so humidity in the air allows static electricity to leave objects.
Static electricity occurs when objects have imbalances of negative and positive charges. When either positive or negative charges on an object become imbalanced, they build up until they find a means of release. The resulting "shock" that people often feel from static electricity is these charges being released and restoring the balance of positive and negative charges.
In dry air, the imbalanced charges have nowhere to go, so they build up until they come into contact with another conductive object that can release the excess charges. However, in humid air, static electricity has a natural means of release. Because water conducts electricity, the moisture in humid air serves as a natural conductor that allows charges to leave objects. Tiny particles of water touch the object and absorb the charge. In turn, those particles of water come into contact with other particles of water, causing the excess charges to spread away from the original object. Higher humidity levels conduct static electricity much easier, as there are more moisture droplets in the air. In addition, humidity can create thin layers of moisture on object surfaces, which helps reduce static buildup.