What is cost?

In accounting, costs are the monetary value of expenditures for supplies, services, labour, products, equipment and other items purchased for use by a business or other accounting entity.

That came out of wikipedia. You can easily Google this, and I'm quite sure the definition is in your textbook if you would look. There are all kinds of different costs. If you need to know about something specific, you're going to have to ask something specific.

And please start your own thread for your questions instead of tagging onto other people's. You question wasn't even related to that post, and I've moved it to its own thread. (Also no need to double post.)