In the poem, the line "The curfew tolls the knell of parting day" refers to the ringing of the curfew bell, which historically signaled the end of the day, marking the time when people were expected to return to their homes for the night.
The curfew bell is also associated with a solemn or mournful sound, often linked to the passing of time, death, and the closing of another day—perfectly fitting the reflective and meditative tone of the poem.