The phrase "bitter rivals" refers to two individuals or groups who have a deep-seated hatred, animosity and contempt towards each other. They are constantly competing against one another for supremacy in their respective fields, vying for the same resources, recognition or power. Their rivalry is intense, often resulting in conflicts, disputes or even physical confrontations. The term "bitter" implies that there is a strong sense of animosity and resentment between them, making it difficult for them to coexist peacefully. In short, the phrase describes two individuals or groups who are locked in a constant state of competition and enmity towards each other.