What it means is:
Buy a booster and it counts as one days payment from the 30 monthly needed to keep an empire or enterprise registered. Of course you also receive the chosen booster, so its a good thing to do, like getting the booster for free.
There is a limit to how many boosters your country or CEO can hold, but its quite large, and some need to be used quite quickly, some last forever until you want to use them. So read each booster page carefully.
One other thing, if you have more than 1 country, an empire, then be sure to buy the booster for your MAIN country as this will increase the whole empires registration, 1 day per booster bought.
There is one, 'super boost' I think, that costs more and adds more registration.
FB players in particular note that this an excellent way of getting your war protection for half price (WP boosters are double price).