Final answer:
Line 12 of Shakespeare's 'Sonnet 130' means that the speaker's mistress is a mortal who walks on the ground like any other person, emphasizing her human qualities.
Step-by-step explanation:
The best paraphrase of line 12 from Shakespeare's Sonnet 130, 'My mistress, when she walks, treads on the ground,' is 'Like everyone, she walks on the ground.' This line intends to convey that the speaker's mistress is a mortal, just like any other human, and does not possess the unearthly qualities or the idealized perfection that is often attributed to women in poetic comparisons. In the context of the sonnet, it serves to ground the description of the speaker's mistress in reality, countering the hyperbolic and unrealistic comparisons typical in love poetry of the time.