The main word and concept you need to know is “daffodils.” You should know what these vivid, yellow flowers of spring look like, and how overwhelming it would be to see a field of 10,000 of them. You should know what they look like when a light wind blows over them, causing them to flutter, dance, and toss their heads, as Wordsworth aptly describes. I’ve placed a few links below that include some daffodil photographs.
Five additional important words may be unfamiliar to contemporary readers, at least in the manner the poet used them.
From the first verse: “vales” are valleys.
From the third verse: “gay” is used to mean “happy,” “lively,” “merry,” or “joyous.” “Jocund” means “pleasant” or “agreeable.” The viewer of the daffodils can’t be anything but happy and joyous at the sight of the field full of flowers.
From the fourth verse: “vacant” here means “empty of thought,” or “listless.” Alternatively, “pensive” means “deep in thought.” From now on, no matter which mood the narrator is in while at home, he/she can think about the daffodils and make himself/herself smile again.