https://stats.stackexchange.com/questions/156471/imagenet-what-is-top-1-and-top-5-error-rate
Your classifier gives you a probability for each class. Lets say we had only "cat", "dog", "house", "mouse" as classes (in this order). Then the classifier gives somehting like
0.1; 0.2; 0.0; 0.7
as a result. The Top-1 class is "mouse". The top-2 classes are {mouse, dog}. If the correct class was "dog", it would be counted as "correct" for the Top-2 accuracy, but as wrong for the Top-1 accuracy.
Hence, in a classification problem with