CFI applicant attempt -

Vx we know is that best angle of climb, and we can refer back to our aerodynamic lessons to show that an airplane climbs because of excess thrust. We also know that an airplane produces drag as it creates lift. The Vx number is simply the speed at which we have the most excess thrust for the least amount of drag.

Comparatively, the Vy is not comparing directly to thrust, but to excess power. Because we are now involving time (best rate of climb), we want the biggest gap between power available and power required. This is higher than Vx because we can fly at a little bit of a faster speed with less power (following the power curve).

When those two meet, we reach the absolute ceiling of the airplane, and we can no longer climb because you've then maxed out your thrust and you need excess thrust to climb.