It is a classic Chef's term like julienne, blancheur, creme anglais and brunoise. It just means boil away the water to thicken the sauce.

simmer until there is less liquid than when you started

Think this has been answered well but a wee tip.Remember to periodically taste what you are reducing as the two main points of reducing are to thicken and intensify the flavour. If you let it go too far down, you may find the flavour a little stronger than you really wanted. Enjoy