Avalanche effect is a property of cryptographic hashing algorithm. If small change(assume 1 bit changes) in the input value leads to a significant change(half of the output bits flip) in the output(hash value) we called it as avalanche effect of cryptography.

That way people can ensure that a person cannot predict the output massage by looking at the changes of the output hash. So this property brings more security for the massage sender as well as the receiver.

Source>>>https://en.m.wikipedia.org/wiki/Avalanche_effect