It’s where you hide your “assertions”.
Please note, because of the way that expectations are implemented, all expectations (eg must_equal) are dependent upon a thread local variable :current_spec. If your specs rely on mixing threads into the specs themselves, you’re better off using assertions. For example:
it "should still work in threads" do my_threaded_thingy do (1+1).must_equal 2 # bad assert_equal 2, 1+1 # good end end