‘Turn down’ is a popular phrasal verb with a number of meanings, which can be used in professional and social contexts. To ‘turn something (or someone) down’ can mean to reject or refuse a proposal or offer. To ‘turn down’ can also mean to reduce something, such as heat or volume, by changing the controls on a piece of equipment. If something ‘turns down’ it means the rate or level of it decreases. Hotels often offer a 'turndown service' meaning a member of staff enters your hotel room to fold back (turn down) the bed linen, so it is ready to sleep in.
Additional examples with 'turn down' are:
A friend has invited you to dinner, but you have a lot of work to finish:
"I'm sorry to have to turn you down. I don't think I will make it to dinner this evening."
An expert on energy conservation is giving some advice on how to save energy at home:
"If you turn down the temperature on the thermostat at home by just 2 degrees, you can save a lot of energy over the year."