A fictional item. The term is sometimes used humorously on message boards when a user has just posted something (usually controversial) that runs the risk of instigating an argument/backlash, i.e. being flamed.
The purpose of the shield is therefore to 'protect' the person from any potential flames that may fly their way.
Person A: Lol final fantasy x sucks
Person B: ...I hope you brought your flame shield.
Person C: I honestly don't see why so many people like (insert popular game). IMO it has a tedious battle system, the difficulty is too easy, and the story is as clichéd as hell. Discuss. *holds up flame shield*