In this poem, Wordsworth expresses his belief that life on earth is but an echo of a more pure and profound heavenly experience we had before birth and that we remember vaguely as children. Later, as we mature, we forget his experience and move further from the divine source. He writes in the ode that

Our birth is but a sleep and a forgetting . . . / Not in entire forgetfulness / . . . But trailing clouds of glory do we come / From God, who is our home.

This connects to Wordsworth's belief stated in "My Heart Leaps Up" that

The child is father of the man

because the child has a "natural piety" that adults lose as the cares of the world press on them.

Mysticism means a direct, unmediated experience of God, one that does not rely on the intercession of priests or religious institutions. Wordsworth says in this poem that he had such mystical experiences as a child interacting with nature. He writes that:

There was a time when meadow, grove, and stream,

The earth, and every common sight,

To me did seem

Apparelled in celestial light

there hath past away a glory from the earth.