I don't think it can be explained both simply and adequately because it's complicated and there are a lot of references. But in a nutshell it explores a number of pagan celtic legends surrounding the Isle of Avalon/Western Isle of the Dead (Glastonbury in Somerset), mostly to do with the earth's fertility (ie for growing food: "the fruits of her body") represented by and/or connected to human fertility (a woman's ability to produce children: "fertility of all mothers") and a male sacrifice, the "Year King" (the song's narrator), to The Goddess ("Mother Earth"). In this worldview death is not the end but the gateway to regeneration ("to die and be transferred into the earth and then for rebirth").
When I was first researching this song I found the following article which explains most of the references (Annwyn, corn dolls, the nineteen maidens etc), but I was struck by how many times actual lyrics from the song are found in the text ("keepers of (the mysteries of) the Goddess", "sacrifice now united", "the waters in (the) rivers and rhynes rises quickly", "the fruits of her body"). I wouldn't be surprised if Steve has an old dead tree copy of this somewhere in his cavernous library.
The article can be found
here, but should come with a warning: pagans are not at all squeamish about going into details