"Rats!" is a small cry of annoyance or disappointment. You drop your ice cream, you miss the last train, you see a typing mistake after you hit send — and you say "Rats!" It is mild. It is a little old-fashioned and a little cartoonish, so young people may use "ugh" or a real swear word instead; grandparents, children's books, and people who do not want to swear still like this one. Nobody in that moment means the animal. You say it as its own little burst, often at the start of the sentence. Though actually, the same spelling in other sentences still means the animals — so the punctuation and the voice tell you which job the word is doing.