To flicker is to flash on and off at random. If your computer screen is flickering right now, you are probably having trouble reading this.
Something that is flickering is flashing on and off in an unpredictable way. A fluorescent light may flicker for a while before it stops working. A flicker is also a quick flash or burst of light, like a glint or a spark. Similarly, flicker can refer to a sudden and short-lived feeling. A child opening a present might have a flicker of excitement before realizing that the box contains only tube socks and underwear.