Imagery is when words are used to describe the five senses: sight, hearing, taste, touch, and smell. In this poem, Spenser uses visual imagery to liken the narrator's emotional state to a ship that is adrift on the open seas. In those times, ships relied on the stars to help orient and guide them in the right direction. In this poem, Spenser has the reader envision a ship sailing without stellar guidance on the ocean on a stormy, cloudy night. He uses words such as the sky "with clouds is overcast" to help us imagine how the scene looks.
Spenser has the narrator liken this image of a ship alone on a perilous seas with no stars visible in the sky to his own fate without his beloved, Helice. The narrator hopes that when "this storm is past," Helice will again become the lodestar of his life. The lodestar is the North Star, the main star guiding a ship across a sea.
We can assume from the narrator's words that he and Helice have had a quarrel. It helps us understand the narrator's extreme feelings of desolation and loneliness by comparing his emotions to the concrete images of a ship drifting helplessly on a dark ocean. These images act as an extended metaphor for the narrator's sense of loneliness.