多分こんな感じとの意味です：
contextではケース毎に設定出来るので便利です（例えばそれぞれのbeforeが違うとか）

```ruby
context "when params are valid" do
  it "redirects to book path" do
    post :update, id: book.id, book: {title: 'title', image_url: 'test.png' }
    response.should redirect_to(books_path)
  end
end

context "when params are invalid" do
  it "raises an error" do
    expect { post(:update, id: book.id, book: {}) }
      .to raise_error ActionController::ParameterMissing
  end
end
```