The definition I first saw on alt.peeves: "users believing that the first software product they use is how *all* software should work from then on."
The idea is that whatever is familiar, no matter how difficult, is going to be what that person wants to stay with forever. The phrase comes from baby ducks (and other fowl) imprinting on whatever they see first after hatching and considering that object to be their mother, even if it's a human or an inanimate object.
In computers, this phenomenon can lead to new products keeping or at least allowing the option of going back to the interface from an earlier version of the product. Another example is people choosing to remap keys or buy a specific keyboard to preserve the key layout they are familiar with.