When you see this badge (of a red and blue symbol with a lightning bolt through the middle) on your Battle Screen, it means you lack the collection power or rating required to participate in the highest league you achieved in the current season.
Example: If you reach the gold league, but then fall back to the silver league, you will see this icon beside your account username on the Battle Screen until you return to your highest league achieved for the season. In this case, you cannot participate in the silver leaderboard this season.
If you would like to learn more on related topics, click on the appropriate link below:
What is Collection Power
Power Rating of Cards
Daily Quest & Season Rewards Chests
How to Earn Daily Focus Rewards and Season Rewards