I am going to disagree slightly with Mrs. Faithorn, as all the spellings I have found for these illuminations say 'Very' Lights after their inventor, the American Edward Very.
Sometimes coloured flares used for signalling emergencies, but more often to provide illumination in trench warfare at night in the form of brilliant white flares. They were not used to help one's own side, but rather to see if the enemy were out in No Man's Land, either working (wiring, digging etc. ) or patrolling, on a raid etc. When the Germans or British had their own patrols or working parties out, they did not use Very lights anywhere near them. The lights did not burn for very long, unlike the German parachute flares. The best defence was to stand stock still until the flare went out, hoping it didn't fall at your feet. Movement while the flare burnt was easily spotted and immediately attracted enemy rifle or machine-gun fire.
This webpage has a song entitled 'When Very Lights are Shining' dating from the First World War. The tune was the one to 'When Irish Eyes are Smiling'.