The speaker describes moods as places. On some days they feel light and lofty, like the stars, living among clouds and birds and looking down on everything. On others they hit "rock bottom," lying on their back, pressed into the dirt with feet rooted in the soil. These two extremes are simple opposites: up and down, sky and earth, high and low.
But the poem's real focus is the third option. "Most days" the speaker sits right in the middle, "where the ground meets the sky." Instead of choosing a side, they "become everything in between." That final line suggests a kind of acceptance. Ordinary life isn't a constant high or a constant low, but a wide space that holds a bit of both.
The mood is calm and reflective. The short lines, lowercase style, and plain words make it feel like a quiet thought rather than a big declaration, and the closing full stop, set apart after a space, gives it a gentle, settled ending.