To turn down means to refuse or reject something that's been offered to you. When you turn down an invitation to a birthday party, you're politely saying no. When someone turns down a job offer, they've decided not to accept it, even though it was available to them.

The phrase suggests a deliberate choice rather than simply being unable to do something. If you turn down dessert, you're choosing not to eat it, which is different from there being no dessert available. A college might turn down an applicant, meaning they've reviewed the application and decided not to admit that student.

You can turn down all sorts of things: requests for help, suggestions, opportunities, or offers. The key is that something was genuinely available, and you chose to say no. Sometimes turning something down is easy, like declining a food you don't like. Other times it's difficult, like when you have to turn down a friend's invitation because you have other commitments.

The phrase can also mean to reduce the volume or intensity of something, like when you turn down the radio because it's too loud, or turn down the thermostat to make a room cooler. In this sense, you're lowering or decreasing something rather than refusing it.