וְשָׁם יָנוּחוּ יְגִיעֵי כֹחַ
What does the word שָׁם refer to in יוֹם שַׁבָּתוֹן and where will the יְגִיעֵי כֹחַ rest?
From the Ya'avetz: Noah sent the Yonah (dove?) the seventh day which was Shabath and it finally found a place to rest. In addition Knesset Yisrael which is meshula (analogic to? symbolized by?) the yonah will find rest in Shabatah. In addition the Gehenom so-called is "closed" in Shabath so the reshayim have a day of rest. My addition: Sham which generally means a place is here used in the sense of a "place in time" .
This fragment is actually a quotation from Sefer Iyov (3:17):
In the original pasuk, the reference is to death (or the grave; but same idea).
In the song, "Sham" refers to Shabbat itself.
I would suggest that the context of the quotation lends itself to (as in Rony's answer) the wicked resting from Gehinnom on Shabbat, and a hint of the final stage of creation, the eternal shabbat: