The expression “rats” generally means the same thing as “nuts” or “sh*t” or “damn.” It is a statement indicating disappointment or frustration at some daily event, which causes you to be somewhat stressed out.
Here I mean “rats” in the other way. That is those nasty creatures that seem to pop up when you least want them. As rats have been here before humans by about 100 million years or so, one could argue they have as much right to be here as we do. (You could say the same thing about cockroaches, beetles, dinosaurs and thousands of other species that pre-date humans).
For as long as humans lived on the Earth, one of the things they have worked diligently at is eliminating rats from where we live. And for as long as humans live on the Earth they will continue to try and rid their homes of these disgusting creatures.