If a hero's gem is glowing, then they are ready to use their Battle Skill
Each hero you equip has a special battle skill that can be triggered in battle. Some skills are attacks that pierce enemies’ armor, others poison the enemy, others heal your team, create power gems or shift the color of gems on the board, and more. There is a huge variety in types of battle skills. Before a battle skill can be used, it must first be charged. A skill is charged each time you destroy a gem that matches the affinity of an equipped hero. Once its skill is fully charged, the gem on its battle portrait will start to glow. To fire a battle skill, simply tap the portrait of the charged hero.In battle, you can preview a hero’s skill by tapping and holding on its portrait.