Shabbath, as the seventh day of the week is intimately related to the sefirah of Malchut, Kingship. This is evidenced by the fact that Shabbat is our testimony to the fact the HaShem not only created the world, but presides over it constantly. How do we do this? We refrain from any and all "productive" action (read: melachah) on Shabbath. In this way we acknowledge that HaShem is still running the show, and we can relax and enjoy in comfort, entirely trusting to HaShem that if we take a day off, we won't suffer any loss.
As we know also, the sefirah of Malchut represents the mouth, speech, and the creation of the world, to which we are testifying through guarding the Shabbath, took place through ten acts of speech. It is for this reason, to allude to the deepest essence of Shabbath, that Masechet Shabbath starts with "Yitzi'ot" Shabbath. The utterances of Shabbat.