King Lear is a play in which an old, wealthy, powerful man realizes he is losing a grip on all that constituted his identity. The opening scene has Lear giving away his property, knowing he is nearing the end of his life. He doesn't have a specific condition or date when he'll die, but he simply feels his demise is not far off.
Lear has vast properties, is highly connected to the powers that be, and has lived a long life. He has three daughters, two older ones (in their forties, it seems) named Goneril and Regan. And, another, somewhat younger (perhaps in her twenties) called Cordelia. The king dotes on the youngest, but leaves practical matters to the two older ones. He dotes on Cordelia, yet relies on the older daughters for management.
Goneril is the oldest. Lear has decided to abdicate his throne and hand his lands to his daughters prior to his death. Had he not, Goneril would have inherited the whole estate as the law stood. This is commentary on what Lear thinks of his children, that he loves them enough to consider their individual needs.
In Act 1, Scene 1, Goneril says to Cordelia, "What shall Cordelia speak? Love, and be silent." It is clear she believe Cordelia's role is to be sweet, a disposition that seems to come naturally to her.
Also from the outset, when Lear asks his daughters to profess their love for him, it's obvious Goneril and Regan are using exaggerated praise, with little sincerity. Cordelia is simply more likable; however, Goneril has reason to be resentful as she would have inherited everything it if weren't for Lear's somewhat madcap plan to divide everything.
Some would say Lear's non-traditional division of lands was a selfish move, perhaps a desire to gain allegiance from all three daughters or a last-ditch effort to show his power. In any case, both Goneril and Regan become caught up in plotting and scheming throughout the rest of the play. By the end, the characters have turned against one another.
Lear is trying to protect his daughters. In his relationship to them, he idealizes them - unable to clearly see the rifts, unable to keep from favoring one (the youngest, fairest, and sweetest), and unable to truly understand their relationships to each other. He is doting, but distant. He is caring, but blind. He tries to exercise power to show himself a father, protector and leader but he is unable to master the forces he's unleashed.
Ultimately, the commentary is about a father's desire to protect his inability to do so in the real world. His actions begin a series of events that bring downfall to all of them. Goneril poisons Regan, her main rival, over jealousy—then kills herself. Cordelia, who was arrested earlier as the sisters and their husbands attempted to undermine her special role, dies in prison. Lear kills himself out of grief.