With a lot of games, and particuarly in this scenario, Magic the Gathering, flavor is an aspect of the game that doesn't directly deal with the game play, but it has a big impact on the overall enjoyability of the game for some people.
If you're wondering what flavor is, it's what makes a card fit into a world or a story. For example, in the history of magic, every single spider printed (except for one very old card) has some kind of hate towards flying creatures. Why? Because spider webs catch flies, of course! Another example of flavor would be Mogg Flunkies - they're bigger than they should be for their cost, but they can't attack by themselves. Why? Well the flavor is they need someone smart enough to tell them what to attack, because they're really, really dumb.
The flavor of things isn't just in the cards, but often in the game itself. Magic doesn't have a deck, it has a "library". There's no discard pile, there's a "graveyard". These are all considered forms of flavor.
My question is: are questions that enquire about flavor permitted on BCG? Example questions might be "What is the flavor of Planeswalkers going to the graveyard?" or "What is the flavor behind Forced Fruition?".