In the first two lines of the poem Wordsworth applies the image of a cloud to himself, which symbolizes his integration with the natural world. Both he and the cloud are aspects of the world, which is subject to the laws of nature but they can still retain their freedom in spite of this. Other images in the poem reinforce this – the dancing flowers, the lake, the breeze and the continuous stars – and they are associated with “self-joy” and contented solitude.
In the stanza, "host" refers to the group of golden daffodils the speaker encounters. In modern English, the stanza might read like this:
I floated along by myself, like the only cloud in an otherwise empty sky, looking at the world around me without really seeing it.
Suddenly, I saw a huge patch of daffodils, a whole bunch (host) of them, all together in a glorious patch of gold.
A way to see the meaning of "host within the context of the original poem is to take Wordsworth's lines out of poetic structure:
"When all at once I saw a crowd, a host, of golden daffodills."
I attached a link that shows the same use of the word host, meaning "bunch, multitude, or crowd" that comes from the biblical book of Luke as using host to mean "group of something" is not used much in modern, conversational English.