Affection is the positive feeling you may have or express for other people or things. Your grandma may show her affection for you by pinching your cheek, and you may show your affection for your dog by rubbing her belly.
Not everybody is good at showing affection to their loved ones, like old-school dads or tough guys in action movies. If people don't get enough affection, they crave it, and will go to great lengths to get it. You might say that affection is what Luke wanted from Darth Vader all along, in Star Wars.