"One eye on the fish, the other on the cat,” (Um olho no peixe e outro no gato), is how the Brazilian say when they are aware of everything what could happen.

Complementando as respostas, aqui seguem alguns exemplos: 1- Em entrevista, o técnico afirmou estar feliz com o desempenho do time na competição, mas disse que é preciso manter-se com "um olho no peixe e outro no gato" (= apesar de o time estar liderando o campeonato ainda há jogos por vir, portanto é preciso estar atento aos concorrentes para não ser ultrapassado e perder o prêmio, no caso, o título de campeão) = In an interview, the coach said he was happy with the performance of the team in the competition, but said that it is necessary to keep "one eye on the fish and another on the cat" (= although the team is leading the championship there are still matches ahead so it is necessary to be careful about other competitors to don't lose the prize, in this case, the title of champion) 2- Para ser um empreendedor de sucesso é preciso estar sempre com "um olho no peixe e outro no gato" (= preocupar-se não apenas em atender às necessidades do cliente, mas também estar atento as ações dos concorrentes) = To be a successful entrepreneur you must always keep "one eye on the fish and another on the cat" (= to worry not only about meeting your clients needs but also pay attention in your of rivalry actions)

Tom. It's simple! Cat loves fish (eat fish), don't they? The expression "one eye on the fish the other on the cat" is used when we do two things at once. So it means that you have to pay attention to what are you doing (watching the fish and also the cat, in the same time) or the cat will eat the fish that you are cooking. Did you get it?

You have to be aware of everything around you, while dedicating yourself to a certain task, so that nothing goes wrong. Cats are clever animals, in a blink of an eye they would quickly eat your fish! So the idiom is a kind of warning against the Murphy's law. So be careful, keep awake and prepare for a fast response just in case it is needed.

Imagine the following situation: someone is cooking a fish. If you neglect the fish, the cat will eat it. If you watch the cat, you can get hurt by preparing the fish. Keep one eye on the fish and the other on the cat. Another variation is: Um olho no gato e outro no queijo (one eye on the cat and the other on the cheese). This is from a mouse perspective. You can use these expressions in any situation where you want to describe that you need to be aware of everything