The quote was fromHOME, Book of Lost Tales Vol 1 - The music of The Ainur. Anyone wanting more detail on the creation should read this chapter at least.
The seas were between ME and anywhere else and Men were unable to sail West to the Undying Lands and many were drowned trying to sail those seas. So they are a barrier of sort controlled by Ulmo with a little help from his friends.
There is also an interesting quote in the same chapter about the 'gifts' to men. The Eldar would be the fairest and the most lovely of all things but Iluvatar gave a new and greater gift to men...
Therefore he devised that Men should have a free virtue whereby within the limits of the powers and substances and chances of the world they might fashion and design their life beyond even the original Music of the Ainur that is as fate to all things else. This he did that of their operations everything should in shape and deed be completed, and the world fulfilled unto the last and smallest. Lo, even we Eldar have found to our sorrow that Men have a strange power for good or ill and for turning things despite Gods and Fairies to their mood in the world; so that we say "Fate may not conquer the Children of Men, but yet are they strangely blind, whereas their joy should be great".
Rumil to Eriol - The Music of The Ainur
It is however of one with this gift of power that the Children of Men dwell only a short time in the world alive, yet do not perish utterly for ever, whereas the Eldar dwell till the Great End unless they be slain or waste in grief..... and dying they are reborn in their children...
There is so much more detail given here about the creation, the relationship between elves and The Valar, the minds of men....... fantastic stuff......... I'm going to burst!