Elevators are methods of vertical travel that go directly up or down, unlike stairs. In vanilla Minecraft, this might be achieved with some sort of redstone contraption, however Civ servers have added ways to make elevators outside of redstone. Early attempts to do this were by using minecarts to teleport players to the highest available block, however you could only use it to travel to only that highest possible point. Servers made using 1.13 and further can also utilize water elevators.
CivRealms 2.0 was the first to have Gold Block Elevators, in which a player would left click to go down or right click to go up to the next available gold block (as long as the space above it is unobstructed). This proved to be a popular mechanic, which led to other servers adding this to their own plug-in list, although with slightly different controls, with the CivClassic implementation being to left click to go down, eventually cycling back up to the highest, and the Devoted Hell implementation being jump to go up, and crouch to go down.