A computer led or lcd screen that flickers could be caused by a few different things.
But first check to be sure that the cable is firmly seated in the connection to the monitor and the computer as sometimes a loose monitor cable can cause screen flicker.
Other causes are the refresh rate is set too high so try resetting the refresh rate on your monitor if the cable doesn't fix it.
If it's an aging monitor then it could be your lcd bulbs going out. If you've ever noticed florescent light bulbs flicker when about to go out the same could be happening in your monitor.
Although newer monitors now use LED bulbs instead which gives it a brighter picture and helps it to last longer.
Your video card could also be the culprit but if it's an older lcd monitor then I would say that the monitor is about to go completely out if the cable connections and video card is good.
If you're using a VGA cable try hooking your monitor up with an HDMI cable instead if your monitor has an HDMI input and your computer has an HDMI output.
HDMI provides a much crisper image on the led monitor and you'll notice a huge difference in the way things are displayed on the web and pictures as well.