Rabbi Yitzchak raised a pointed question about the relationship between two sacred "signs" in Judaism: the Sabbath and tefillin (leather phylacteries worn during prayer). Both are explicitly called a "sign" — an ot — in the Torah. The Sabbath is a sign of the covenant between God and Israel. Tefillin are a sign upon the hand and between the eyes.
Rabbi Yitzchak's teaching was that one sign should not be superimposed upon another. Since both Sabbath and tefillin carry the designation "sign," wearing tefillin on the Sabbath would be redundant — piling one sign on top of another when one already suffices. This became the basis for the widespread practice of not wearing tefillin on Shabbat (the Sabbath).
But the Mekhilta considers a counterargument. Perhaps the two signs are not redundant at all. Perhaps they serve different functions and should both be observed simultaneously. Rabbi Yitzchak dismantled this objection with a simple observation: the Sabbath is called not only a "sign" but also a "covenant." Tefillin are called only a "sign." Since Sabbath carries both designations — sign and covenant — it holds the stronger claim. The weightier sign overrides the lesser one.
This teaching illustrates a sophisticated legal principle. When two commandments overlap in their symbolic function, the rabbis looked to the Torah's own emphasis to determine which takes precedence. The Sabbath, bearing the double distinction of sign and covenant, wins the conflict. Tefillin step aside on the day that already proclaims, with even greater force, the bond between God and Israel.