Detrimental is a formal way of saying "harmful." Anything detrimental hurts, hinders, or puts a damper on something. Detrimental things do damage.
Have you ever heard "Smoking may be detrimental to your health" and wondered what it meant? It means that smoking is bad for your health: it's going to harm you in a bunch of ways. Detrimental is a more official, formal way of talking about things that do harm. A dog who likes digging holes can be detrimental to a garden. If you're writing a paper and want to pick an alternative to harmful that sounds more formal, detrimental is a great choice.