The Rebbe once encapsulated the entire Rosh HaShanah into two succinct points:
The idea of the entire 48 hour period of Rosh HaShanah is two points—one from below and one from above.
The point from below is that a Jew subjugates himself to G-d. He accepts upon himself the yoke of G-d’s kingship.
The point from above is that G-d—so to speak—subjugates himself to fulfil the verse (Vayikra 26:4) “I will give your rains in their time…” and the rest of the various blessings that are expressed in that section of the Torah.
 Likutei Sichot. Vol. 2 Pg. 406