This quote is from the same Druidry book as last week's post, and is about the different worlds, or realms of existence, according to the Ancient Celts:
From Brendan Cathbad Myers's The Mysteries of Druidry (2006):
"The threefold 'vertical' division of Sky, Land, and Sea, which roughly correspond to Heaven, Earth, and Underworld, is intersected with a fourfold 'horizontal' division of the Earth, the realm where we live, corresponding to the four cardinal directions."
The terrestrial, mortal world that we live in is called Abred. It seems to me that this is the only world where things can grow, die, and actually change. Obviously, the symbol for this is the land.
Lastly, the celestial realm, where spirits and deities exist, is called Gwynfyd. This is our proper home, and our soul is on the way there, though it may go between Abred and Annwin many times as it incarnates into physical form and then becomes a spirit (bodiless being in Annwin) in an intermediate period between its next incarnation. This is the Druid's idea of reincarnation. Similarly to many other religions such as Buddhism, the soul will eventually get out of this cycle to reach Gwynfyd (or at least, that's the goal). The number of reincarnations it takes to reach Gwynfyd is different for everyone due to their level of enlightenment, making some people "ready" for the higher world earlier than others.
These realms are united along a vertical axis, conjoining in a Sacred Centre on Earth, symbolized in the image of the World Tree. The Druids made passage mounds such as at Newgrange in Ireland (see picture) to unite these realms and so be able to pass between them or communicate with beings in the other worlds. At Newgrange, sunlight enters the inner chamber only once a year on December 21 (Midwinter's Day), where the sun is "born" anew. This is when the three realms come together at the centre, and life is renewed every year. So the Druids' idea of "creation" is not a single process, but rather something that is ongoing that we can take part in as well.
In the picture below, you can see the three worlds (named differently though) and the four divisions of the Earth: