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Obligate aerobic means that the organism needs oxygen for its metabolism and cannot survive without it. There are anaerobic organisms that do not need oxygen, and for some of them the supply of oxygen is even poisonous, and we speak of obligatory anaerobes.
Explanation for children:
Humans are obligatory aerobic, which means that we absolutely need oxygen (air) to survive. Other organisms, such as intestinal bacteria, can only survive without oxygen, so they live deep inside our intestines.