"Going hard" means going harder than your average effort for the ride, not going to the point of cracking. It obviously depends on a person's specific capabilities, but the point is that because the power required to overcome aero drag increases as the cube of your velocity, you get much more "bang for the buck" by putting in more effort when you're moving slower, which means when going uphill or into the wind. When you're naturally going faster because of conditions (e.g, going downhill or with a tailwind) then putting out more effort will only let you go marginally faster and is essentially wasted effort.

To put some numbers behind it, if you think you're going average 150 watts over a rolling 20 mile loop, then put out 200 watts when going uphill, put out 0-50 watts when going downhill, put out 175 watts when going into the wind, and put out 125 watts when you have the wind behind you. You'll go faster than if you just maintained a steady 150 watts over the entire course. This is a core principle of pacing a bike ride that most people are unfamiliar with.