Rosh Hashanah as the source before Divine flow divides

ראש השנה פי' קודם התחלקות השפע להיות נשתנה.

"Rosh Hashanah" means [the point] before the abundance (shefa) divides up and becomes transformed.

The Sefas Emes explains the inner meaning of Rosh Hashanah: it is the moment before the Divine flow of shefa branches out and undergoes change into the varied forms of the world.

כי כשבא חיות השי"ת לעוה"ז תחת הזמן והטבע משתנה מרוחניות לגשמיות.

For when the vitality (chiyus) of Hashem, may He be blessed, comes into this world, under [the domain of] time and nature, it changes from the spiritual to the physical.

As Hashem's life-force descends into our world of time and nature, it is transformed from a purely spiritual reality into a physical, garbed one.

ור"ה הוא המקור וההתחלה קודם ההשתנות כי במקורו הוא בלי ציור גשמיי:

And Rosh Hashanah is the source and the beginning, before that transformation, for at its source it is without any physical form (tziyur gashmi).

Rosh Hashanah is the root and starting-point of the year, standing before the chiyus is clothed in physicality — at this source it remains pure, beyond any material shape. This is why it is the wellspring from which the whole year's life flows.

Summary: Rosh Hashanah is the source and starting-point of the year — the moment before Hashem's Divine vitality divides and descends into the forms of time, nature, and physicality. At its root the chiyus is still purely spiritual, without any material form, which is why Rosh Hashanah is the wellspring of the entire year.