The "inward eye" is the insight of the mind and spirit that has recorded an experience and delights in its beauty and inspiration later when recalling its memory.
In his essay on "Nature," Transcendentalist (precursor to an American Romanticist) Ralph Waldo Emerson writes that when he is out in nature, he becomes
...a transparent eyeball. I am nothing. I see all. The currents of the Universal Being circulate through me....I am the lover of uncontained and immortal beauty.
So, too, does the speaker of Wordsworth's poem experience the "uncontained and immortal beauty" of the daffodils. His memory records the delight that he experiences in the lovely sight of the golden daffodils whom he personifies as a "jocund" company that toss their heads and dance. Indeed, their freedom and beauty inspire him and the image of this romantic delight in nature remains for him to recall when he lies on his "couch" at home and reflects upon the enjoyment of the beauty of Nature that he has previously experienced.