Leviticus is a guide through the rituals and rules of the priests serving in the Mishkan, taking us ultimately to the truths found in the holy of holies. As we begin our journey we hear God calling to Moses, asking him to enter the Mishkan.
The LORD called to Moses and spoke to him from the Tent of Meeting.
Rabbi Henach Leibowitz
[The final aleph in Vayikra, וַיִקְרָא , is written smaller to signify Moses' humility.] Haughtiness, insolence and pride are actually derived from the opposite feelings: inferiority, insignificance and shame. One who feels confident in himself has no need to deprecate others or to represent himself as something other than he truly is. ... the essence of humility is the realistic understanding of one's own worth.
Midrash Rabbah Vayikra (1:15)
Another interpretation: He called to Moses and Hashem spoke to him from the Tent of Meeting saying, From here [the Sages] derived a teaching and said: Any Torah scholar who lacks wisdom [daat], an animal carcass is better than he. Know that this is so. Go forth and learn from Moses, the father of wisdom, the father of prophets ... he did not enter the innermost chamber of the Tabernacle until [God] called him ...
Rabbi Shlomo Wolbe
What exactly is daas and why is it so important? Daas is an awareness of what one is doing, as opposed to performing actions out of habit or mechanically. With regard to davening, daas is referred to as "kavanah." ... Yishuv hadaas describes actions performed with an awareness of what one is doing. [Yishuv hadaat is typically translated as a "settled mind" or equanimity.]
Rabbi Avraham Greenbaum (as cited by Rabbi Alan Lew, This is Real and You are Completely Unprepared)
... the ability to sit calmly is one of the most important prerequisites of clear-headedness. The Hebrew term for a calm, settled state of mind -- yishuv ha-dat -- is bound up with the concept of sitting. Daat is awareness or consciousness, intellectual, emotional, meditative, intuitive or all of the above. ... Yishuv hadaat means to settle the mind. ... sitting in this sense not only signifies the physical act of seating the body quietly. It also suggests the whole principle of taking a break from the activities of day to day living in order to fully inhabit our lives. The Hebrew word for sitting is thus connected with the idea of Shabbat, when we pause and sit back from our workaday activities in order to cultivate the spirit, to inhabit our lives with spirit and consciousness.
Rabbi Avraham ben Ha Rambam, Guide to Serving God
Know that calmness and humility are partners, just as their opposites, anger and pride, are partners. Generally, what stirs anger is inward pride, while what causes calmness is inward humility.