When Lear says "Blow winds and crack your cheeks! Rage, blow!" he is calling upon the storm to do its worst, both to him and to the world. One can tell that he is doing both because of what he says later in the speech. He calls on "you sulphurous and thought-executing fires"—that is, lightning bolts—to attack him directly and "singe [his] white head." He is not calling down the storm's wrath only upon himself, though. He also asks it to "strike flat the thick rotundity of the world," to crush the entire globe in its fury. Lear wants the storm to "crack nature's molds. All germens spill at once/That makes ingrateful man." This line means that he is asking it to destroy the human race altogether. He specifically demands that it do so not by killing all the humans at once, however, but by killing all the fertile women specifically; women are "nature's molds," filled with metaphorical seeds or "germens," that will "make ingrateful man" if they are not destroyed. Lear's rage against women in particular makes sense because his daughters have just turned him out into the storm. When he says "blow winds," etc., he's beginning a crescendo of fury against himself, but also against everyone and everything around him.

The angry and bitter Lear, who has been betrayed and humiliated by his daughters, goes outside into a storm with his Fool. He knows now that all the words of undying love and loyalty offered him by his older daughters were lies.

When he shouts to the wind to "crack" its cheeks by blowing and raging, he is personifying the wind. To personify an object is to give it human attributes. He is picturing the wind as an angry man puffing his cheeks and blowing hard. (When the weather reflects the mood of a person in a work of literature it is called the pathetic fallacy: here the violent storm reflects Lear's violent rage.)

Lear wants the wind and the storm to express his emotions. He is now powerless and can't act on his rage in any effective way. However, the weather still has power, and Lear wants it to use its power to punish humankind. He ends the speech by calling on the "thunderbolts" to:

Smite flat the thick rotundity o' th' world,

Crack nature’s molds, all germens spill at once

That make ingrateful man!

There are a couple of layers to meaning in the lines that help to establish the second scene of the Third Act.  The reality that has dawned upon Lear is that he is no longer the power broker that he once saw himself to be.  Rather, he is a political tool being manipulated by his two daughters as they seek to "rage" and ravage through his political empire.  The lines that Lear utters help to bring about Lear's own transformation in power.  As opposed to the powerful image at the start of the drama, Lear recognizes his diminishing power.  In this, the lines bring out the idea that the power of the storm is something that can prove to be overwhelming to Lear.  At the same time, a theory of correspondence between the actions of the characters and the weather conditions is evident in the lines.  Lear invokes the power of the destructive storm to bring out how the danger in human actions is evident.  The invocation of the storm is a dramatic device in which Shakespeare foreshadows the destructive actions of the human beings.  The "crack" of cheeks as well as the command of "Rage" and "Blow" help to emphasize how there is to be some level of destruction wrought at the hands of human beings to be evident.  In this, the lines help to demonstrate Lear's own predicament of emerging powerlessness and the moral abyss that the characters are about to enter.