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!