The best rate of climb speed Vy will cause one to gain the most height in a given time period.
Imagine you are taking off with a 610ft hill 1nm ahead.
If the aircraft climbs at 60Kt (Vy) and tghis gives a rate of climb = 600ft/min the aircraft will be 600ft at the hill and will impact 10 ft below the top.
The whole idea of flying at the speed for best angle is to sacrifice a small amount of climb rate in return for a (proportionally) bigger reduction in speed.
Let's say that we now fly the same example at the best angke speed of 50Kt(Vx). This gives us a reduced rate of 550ft per minute.
However, even though we are climbing at a reduced rate it now takes longer to travel the 1nm to the hill and you can simply work it out that we will now
travel 5/6nm in 1 minute (climb 550ft)
and then climb 110ft in the remaining time it takes to travel the 1/6nm left to the hill.
Therefore despite climbing at a reduced rate we are now at 660ft as we safely pass over the hill.