The light bulb is an icon of a hero's skill. You can find each hero's skill in the hero detail screen. Each skill is unlocked once Hero reaches level 5, then it can be upgraded with skill points.
What a skill does at a hero detail screen

When assembling a team of heroes to enter a mission, light bulbs of different colors are used to indicate the status of their skills.
  • Red crossed bulb - passive skill cannot be used, its requirements are not met
  • Orange crossed bulb - passive skill can be used, but it will not, since another hero in the team has the same skill, but stronger, so its skill will be used instead (skills are not stacking)
  • Blue bulb - passive skill can and will be used
A status of heroes skills is displayed when entering a mission