"Customer service" is the name for the group of employees in a company who answer customers' questions, listen to their complaints, and help them out with problems. In large companies, it's a separate group with a special telephone number that you call if you need help.
Here are some situations in which you might call customer service:
- You got a telephone bill and the charges were a lot higher than usual. You don't think you should have to pay that much.
- You want to cancel your subscription to a magazine.
- You ordered a book on the Internet two weeks ago and it hasn't arrived yet. You want to know why it hasn't come.
- Your TV isn't receiving any cable TV signal, so you can't watch TV.
"Customer service" also means whatever actions a company's Customer Service group does for you. So you can talk about customer service as a group of people, or as a thing:
Customer service is always super helpful whenever I call.
They didn't even offer to replace it? That's not very good customer service, is it?