The Otherworld is related to Eternity and best explained by Caitlin Matthews in her book, 'The Celtic Tradition':
"Perhaps more than any other people, the Celts have always cherished the country of their true home -the Otherworld. It is the source of their wisdom, the place of their gods, the dimension in which poets and wanderers are most at home. Whoever has visited the Otherworld becomes more than mortal... The realms of the Otherworld are of the ever-living, where everything is possible, where great deeds are accomplished...".
Hence, the Otherworld is the dwelling place of the Heroes, the Blessed ones whose qualities made them superior. It is AVALON, the "Apple-isle", a paradise across the sea, where the gods and heroes feed on apples of immortality.
As once I was so am I now.
For evermore a hope unseen,
Betwixt the blossom and the bough.
Samhain is the celebration of the Life which never dies. It's the time to harvest the fruits and to keep the seeds to initiate a new cycle. The celebration of the never dying spirit is expressed in many ways at Samhain: Apples and nuts are associated with Samhain and Halloween, as symbols of Eternity and hidden Wisdom. The Apple is the fruit of the Otherworld growing in Avalon. When cut crosswise, the apple displays 5 seeds embedded within a five-branch star, symbol of the Welsh Sow Goddess Cerridwen (the Morrigu) and of the spiritual quintessence ("fifth essence"). The apple then came to be known as the "fruit of the Gods". Apples and nuts are also found on the "silver branch" carried by the poets, or fili, since poets draw their inspiration from the ever fertile Otherworld. Another reminder of Eternity at the door of the dark season was the mistletoe. The sacred mistletoe grows upon the oak, drawing its life-force from the essence of the King of the trees. "At Samhain, a sheaf of corn, a branch of evergreen or mistletoe symbolically carried on the dying powers of vegetation. Carrying or decorating with evergreens demonstrates that life has not died.
The invisible realm is the "Underworld"quite distinct from the Otherworld. This element of the Celtic cosmology rooted in older native beliefs was not conceived of as a hell or place of punishment. It was the well of the primal forces of Life, before and after creation; the place where that which was not yet encountered that which was not anymore. It is the realm of the ancestors, exemplified by the ever turning wheel, "the mill in which the gods of the underworld reside, in which the dead are remade, and initiates reborn.
The different worlds -visible and invisible- are not separate; there are bridges. Many gifts pass between mortals and the Otherworld folk: the power of healing, the power to sing, to make music or create poetry. The faery mounds, or sidhes, are the homes of the Otherworld beings, still known in every Celtic country as the Little people: the brownies, the elves, the fairies, the corrigans, the goblins, the gnomes, etc.
As the cold was intensifying, bonfires were lit. Their purpose was double: encourage the Sun as the Life-giver (sympathetic magic), and draw upon the elemental quality of fire, as an agent of purification. As at Beltain bonfires, people jumped over them and cattle were driven through them; it was a way of getting rid of evil influences, but also of ridding the cattle of parasites.