Answer:
They hide by standing around in herds. The stripes of each zebra are distinctive (like fingerprints on humans), but when you have a herd standing together and all the animals are striped it makes it harder for a predator to focus on and single-out an individual animal for attack. It appears as a giant screen of stiping.
Depending on the grasslands, just standing still in tall grass may make a zebra's stripes effective camouflage.