A “yoke” is a heavy wooden bar attached to the heads of two oxen, horses, or mules so that they can pull a cart or plow. It is designed to limit the animals’ mobility so that they walk together in the direction in which the farmer wants them to go. A yoke is extremely heavy and implies a great burden or difficult task.
“Fondly” in this time meant foolishly. The speaker immediately apologizes for the question that he has posed by calling himself foolish. In a way, this shows the speaker recant his question.
“Light denied” in this context means blindness. Notice that there is an undercurrent of blame in this metaphor. “Denied” suggests that someone took his sight. This reveals both the speaker’s anger over losing his sight and his inability to express this anger at God.
“Spent” can either mean “passed,” as in, "when I consider how I have spent my days," or it can mean “gone,” as in, "when I consider that my sight is gone."
In this context, “light” is a metaphor for both the speaker’s life span and his sight. Since this poem is called “On His Blindness” and we know that Milton went blind in 1652, “light” can be read throughout the poem as a conceit for sight.
