For RSpec 3+:
The message could be customized as a string or using a proc(check the reference).
expect(1).to eq(2), 'one is not two!'
Customized message
RSpec tries to provide useful failure messages, but for cases in which you want more
specific information, you can define your own message right in the example. This works for
any matcher other than the operator matchers.
source @ relishapp
For older RSpec versions
should and should_not take a second argument (message) that overrides the matcher’s default message.
1.should be(2), 'one is not two!'
The default messages are usually pretty useful though.