Zachor and shamor: two levels of Shabbos kedushah

בענין זכור ושמור שמור הוא הקדושה שיורדת בשבת וצריכין לשמור שלא לחלל הקדושה ולכן כתיב בה כאשר צוך ה' אלקיך.

Regarding "Remember" (zachor) and "Guard" (shamor): "Guard" is the holiness that descends on Shabbos, which we must guard so as not to desecrate that holiness; and therefore it is written concerning it, "as Hashem your God commanded you" (Devarim 5:12).

The two versions of the Shabbos commandment — "Remember" (in Shemos) and "Guard" (in Devarim) — point to two dimensions. "Guard" refers to a holiness that comes down from above on its own; our task is the guarding work of not violating it, which is why this version is phrased as a command to protect the mitzvah.

לשמור מצות השבת.

To guard the mitzvah of Shabbos.

The aspect of "shamor" is essentially protective — keeping the laws of Shabbos so that its given holiness is not desecrated.

אבל זכור הוא הקדושה שצריכין בני ישראל להמשיך בשבת תוספות קדושה ע"י חידוש היום ותפלת שבת והוא מדריגה שלמעלה.

But "Remember" is the holiness that Bnei Yisrael must draw down on Shabbos — an added holiness — through the renewal of the day and the prayers of Shabbos; and this is a higher level.

"Zachor" is the active, ascending dimension: rather than merely guarding what is given, Bnei Yisrael actively draw down an extra measure of kedushah through experiencing the day anew and through the special tefillos of Shabbos. This is a loftier avodah than guarding.

אבל אחז"ל כל שישנו בשמירה ישנו בזכירה שע"י בחי' שמירה זוכין למדריגת זכור:

But Chazal said: "Whoever is included in guarding is included in remembering" — for through the aspect of guarding one merits the level of "Remember."

Chazal's principle (Berachos 20b) links the two: one cannot reach the higher, active level of "Remember" without first fulfilling the foundational work of "Guard." The guarding is the gateway through which one earns the higher drawing-down of kedushah.

Summary: The two Shabbos commandments express two levels of holiness — "Guard" (shamor) is the protective work of not desecrating the kedushah that descends on Shabbos, while "Remember" (zachor) is the higher, active drawing-down of an extra holiness through the renewal of the day and its tefillos. Yet the two are bound together: only through the foundational guarding does one merit the loftier level of remembering.