The speaker narrates a journey that he takes towards Lucy, the object of his passion when she is looking as "fresh as a rose in June." As he travels towards her home, he spends his time looking at the moon in the night sky and contemplating its beauty.
As he travels, the moon continues on its course, and the way in which it is described as descending in the last three stanzas prompts the speaker to experience some kind of premonition or feeling of unease where he engages in a fantastical thought. As he finally watches the moon go behind Ludy's cottage he experiences what he calls a "fond and wayward" thought that perhaps Lucy, the object of his affection, has died. This of course is the "strange fit" that the speaker refers to in the first stanza.
The poem therefore captures the way in which we all can experience "strange fits of passion" in our intense love for another person. The speaker refers to the way in which his sense of anticipation in reaching his lover's cottage is somewhat marred by this "strange fit" that could take his lover from his embrace.