Imagine you are working on the sales data of a website. You know that 99% of website visitors don’t buy and that only 1% of visitors buy something. You are building a classification model to predict which website visitors are buyers and which are just lookers.
Now imagine a model that doesn’t work very well. It predicts that 100% of your visitors are just lookers and that 0% of your visitors are buyers. It is clearly a very wrong and useless model.