Translation:They are neither at home nor in the forest.
The -kä suffix is a coordinating conjunction that is attached to the negative verb ("eikä", "eivätkä"...) when the coordinate subordinate clause is negative.
"En käy kalassa enkä marjassa." - I go neither fishing nor berry picking.
The -kaan/kään suffix is the negative counterpart of -kin.
"Minäkin juon vettä." - I too/I also drink water.
"Minäkään en juo vettä" - I don't drink water either (me neither)
"Minä juonkin vettä." - I'll drink water instead (not milk)
"Minä en juokaan vettä" - I'll not drink water after all.
"Minä juon vettäkin" - I drink water also (not just milk)
"Minä en juo vettäkään." - I don't drink even water (this is a bit of a clumsy translation, haha)
This explanation doesn't cover all the uses of -kin and -kaan/kään, but just know that they are used to create nuance and emphasis. You can also see them in fixed expressions such as "ainakin" (at least), "ainakaan", "sittenkin" (after all, nevertheless, yet), "sittenkään", "joskin" (even though, albeit), "joskaan" etc.
It seems odd to me that -kA is considered a coordinating conjunction as opposed to subordinating. With either ... or the two things compared have an equal status -- they are each marked as being in this relation. With -kA you have one unmarked constituent and one marked. He eivät ole kotona can stand on its own; eivätkä metsässä cannot. In other languages I can call to mind -- I'm thinking of Latin, German, and Welsh -- the neither ... nor construction also involves marking of both constituents, like English.