What does this mean?

It means you get can expect something like the output listed for the Empire in this table

The key word is anechoic. That typically means not in room or measured in a chamber that is meant to take the room out of the equation. In a room you should get lower extension due to room boundaries and an increase in SPL because of the same reason.

Specs wise the sub will do 20 hz, depending on the room it may be able produce a lower range than 20 hz.

Kinda like a car, with a max speed of 200mph or faster up to 250 depending on road conditions and if there is a hurricane pushing the car