English is well-known for being a very idiomatic language with lots of old expressions and new ones being born all the time.
Let’s take two quick examples and at the bottom we’ll provide links to some good sites.
Cash cow: A cash cow is a product which has a low investment cost for the company but easily brings in a lot of money. Sometimes people say slot machines at casinos are a type of cash cow.
Go back to square one: This is an expression which means to start over from the beginning. Example: After weeks of negotiations everything fell apart and we couldn’t come to an agreement. We’ll just have to go back to square one.
There are many business idiom sites out there. All of these looked good to us:
http://www.idiomconnection.com/business.html (has free quizzes too)