To blink is to briefly close your eyes. The bright sunlight first thing in the morning might make you blink rapidly, until your eyes adjust.
Your eyes blink reflexively, when you're not even thinking about it, but you can also deliberately blink back tears. Certain lights blink as well, like your car's turn signal, flashing on and off. A figurative way to blink is to hesitate or give up, especially at a confrontation: "The lawyers kept holding out for a settlement, waiting for the other side to blink first." The origin of blink is probably the Middle Dutch blinken, "to glitter."