To pinch is to sharply squeeze or grip with your fingers. No matter how much you love your grandma, you probably don't enjoy it when she pinches your cheek and tells you how tall you've gotten.
You can pinch your little brother when he's being annoying, and you can pinch an image on a touchscreen (to make it smaller or larger) by moving your fingertips the same way. Pinch is also a noun, meaning the act of pinching or the amount of something you can hold between your finger and thumb: "Add a pinch of salt to the soup." Informally, if you pinch something from a store, you steal it.