The third quatrain of Shakespeare's poem features imagery from the world of agriculture. On the one hand, "beauty" and everything connected with it starts to be described in terms of growth and the natural world. On the other hand, this refreshing imagery only comes up in the context of learning how it is destroyed—by time who becomes personified as the farmer who plows—and thereby tears up—fields, and who eats and mows the produce of the earth. This agricultural imagery could be taken as giving a hint of rebirth, but Shakespeare doesn't emphasize that point. Instead, he emphasizes the death of everything that is ever born.
Lines 8-12: Throughout quatrain 3, Shakespeare uses personification in the way he describes time performing various destructive activities: he "doth transfix," he "delves," he "feeds," and "mow[s]" everything that stands. This use of language appropriate to human beings to describe an abstract entity like time makes it easier to focus attention on how negative some of time's effects can be. In quatrain 3, time is one evil dude.
Lines 9-12: The personification isn't all that clear in Line 9, mainly because the language is a bit hazy, when it talks about how time "transfixes the flourish set on youth." But a definite picture emerges when we learn how time "delves" lines in people's foreheads, "feeds on" what nature produces, and "mow[s]" everything that grows. The last image especially makes time almost sound like the grim reaper, don't you think?
Line 10: In this line, Shakespeare uses a vivid metaphor to capture the destructive power of time. By saying that time "delves the parallels in beauty's brow," it becomes clear that he is comparing "beauty's brow" to a field, in which time is digging trenches.