When I behold, upon the night’s starred face,
Huge cloudy symbols of a high romance,
And think that I may never live to trace
Their shadows with the magic hand of chance;
These lines from Keats' "When I have Fears" captures the speaker's fears of dying before he can accomplish everything he desires. In this particular section, Keats uses imagery and metaphor to discuss two things he fears not being able to do before death. The first act is the literal and obvious reading of the poem, which is to say that the speaker wants to experience the natural wonder of the night sky and stars. As a Romantic poet, Keats finds peace in the natural world.
The speaker of the poem also makes a connection between the starry night sky and "high romance;" in the previous section of the poem, Keats laments that he may die before he can write all of his poetry ideas from his "teeming brain."
Now, as he creates the imagery of the beauty of the night sky and the "huge cloudy symbols of high romance," Keats also suggests that part of his ideas for writing also concern romance. The "huge cloudy symbol" is Keats' way of expressing his desire to write a long or perhaps important love poem. 'Huge' could refer his longed for romance poem's importance or content, and the cloud-like description suggests that his inspiration or ideas for the poem float around in his "teeming brain" like clouds.
The notion of him "tracing the shadows with the magic hand of chance" is his fancy and poetic way of describing how he will capture the star-like mystery and appeal of Romantic love; he compares the art of recording those thoughts and dreams to the slight of hand of a magician.