This is a metaphor. Time does not really have a chariot, but the speaker uses this image to show that time never stops. It always moves forward and takes people closer to old age and death.

Here, these lines change the mood of the poem completely. Before it, the speaker talks about how he would love the lady slowly, if they had enough time. He would spend hundreds of years praising her eyes, breasts, and each part of her beauty. But this is not possible because time is moving fast, and soon they will be too old or dead. He warns her that after death, her beauty will be gone. And they will not be able to enjoy love. The speaker suggests they should enjoy love and life while they are young and fresh.

So, these lines are the speaker’s attempt to convince his “coy mistress” to enjoy love with him before time runs out. Marvell uses it to say that life is short, so we must live fully and love while we still can.