On a superficial level, Wordsworth is simply referring to the beauty of the field of daffodils. He appreciates the brilliant yellow color against the background of the trees, the reflection of it all in the water, and the motion of flowers and waves in the wind. At the beginning of the poem, he had apparently been feeling somewhat lonely, but by the end of the poem he has a joyful memory of that sight to cheer him.
Looking deeper, in the first stanza the poet is detached from the surroundings; perhaps ill and unable to get out, perhaps depressed and choosing to withdraw from society? The sight of the daffodils gives the speaker a vision of beauty and delight that is in great contrast with his frame of mind - "A poet could not but be gay,
In such a jocund company." The "wealth" of that experience stays with the poet and carries him back to a beautiful and pleasurable memory when he has returned to his "vacant" or "pensive mood." The memory of the daffodils allows for some escape from the isolation of his condition, whatever it is.