Question

Here is the question,
Compare the human/chimpanzee to each other organism, entering the number of differences in the chart. Note that a minus sign (-) indicates that an amino acid is missing in that sequence. When comparing two sequences, an amino acid in one sequence and a (–) in another would count as a difference.

The thing about this question is, I found the answer on this website but I want to understand how to fully understand this. The assignment I am doing makes no sense. I just need help. Can someone explain this to me? But in the simplest possible way so I can understand???