Imagine if every time you went to get a haircut your stylist did things differently. One time they washed your hair, another time they didn't, a third time they swapped scissors for an electric razor with no explanation. Even if your haircuts were equally good, the inconsistent experience would feel uncomfortable.
Really this is about setting and then keeping expectations. By following the same process for every haircut a stylist sets and then meets expectations.
This applies to software too. One classic example is the profile photo. Does tapping a photo always do the same thing? It probably should, but at Google, we often broke this consistency accidentally, especially across apps. Getting this right was one of the small details that helped bring consistency to our experience.