To understand the movement of this setting and how each Epoch relates to the others, one has to have a fairly in-depth knowledge of the cosmology of the mutliverse. I’ll state it simply and succinctly here.
Imagine all the various dimensions as a set of nesting dolls (an infinitely large set of nesting dolls). Our universe is one doll, on either side of us, there are other dimensions, these are our adjacent, or neighboring, dimensions. It is possible (though not easy) to move from our dimension to a neighboring dimension. Many of the same basic laws of physics apply in these dimensions, and while things are by no means the same, they are similar enough that you won’t immediately die or go insane.
On one side, we have what is often called the Glimmer, Feywild, or Faerie. This is a place of magic and untamed wilderness, chaos, and extremes. It is also home of the Faeries. On our other side, we have the Gloom, also known as the Shadowfell or the Nightlands. Creatures of darkness that invade the nightmares of humanity can usually trace their origins to the Gloom.
In addition to these two dimensions, there are the far dimensions, which are next to them. It is possible to travel to these dimensions, but only with great difficulty. Usually it is impossible for physical entities to travel there, thus only spirits can reach these dimensions. On the far side of the Gloom is the Chaos, also known as hell or the abyss. This is an unstable hellscape of flames and ice and acid and whirlwinds. The entities most commonly called demons originate here. On the far side of the Glimmer is the Aether, also known as heaven and the Astral Sea. Here entities of great power forge island-like dominions from the great expanse of light in which they reside. Their servants are often called angels.
Got all that? Good, because it’s mostly wrong. But given that you can’t understand multidimensional algorithmic spacial patterns, it’s about as good an explanation as you’ll get.
One of the key areas this explanation falls short is in the area of parallel dimensions. Parallel dimensions are so similar to one another that they effectively occupy the same dimensional space. In parallel dimensions, all the basic laws of reality are the same, and the only differences are “cosmetic”. I.e. historical events and choices people make. Though these things seem very significant to us, on the cosmic scale, who won what war really isn’t even a drop in the bucket.
As a final, very important, note, between and beyond all the dimensions is what is usually referred to as the Outer Darkness. Little is known of it or of the strange entities which dwell there, known to most simply as Outsiders.