Flicker is the change in brightness of light. The above graph shows how even though incandescent and fluorescent bulbs might flicker at the same frequency (Hz), the increased flicker % in fluorescents is what causes the new distress from flicker.
LEDs are called a Solid State Lighting (SSL) technology. This lighting technology can drop in intensity down to zero in the AC cycle! This is one of the greatest reasons why early LEDs are rejected as a poor choice for healthy lighting.
LED flickering can be tied back to the driver component within the lamp. The essential purpose of the LED driver design is to rely on a simple circuit to control output current, but without altering the frequency, the LED becomes likely to show visible flicker. However, if the driver design of the LED bulb meets the requirements of both a stable DC current and ample ripple suppression, there should be no flicker. Unfortunately there are companies that care more about their bottom line than the health of their customers.