FOR EXAMPLE:
Let's say you've been taking melatonin every single night to help you sleep. After a while, you may notice that it begins to take more and more melatonin for it to work like it did the first night you took it. You were taking one per night, but you end up taking 3 per night to get the same effect because your body has resistant to the drug.
ANSWER:
After a while of taking antibiotics, microorganisms in your body may become resistant to the antibiotics (which is potentially dangerous). This occurs when bacteria change to reduce the effects of a drug.