The concept of fairness is the idea that the two teams competing in a game of quizbowl should have equivalent opportunity to receive points and ultimately win. For a game to be fair, both the questions and the format must be fair.
In a perfectly fair setting, the team which has more knowledge should always win the game, but this would require teams to play an infinite number of perfect questions to remove all variance. Thus, things are typically considered fair if there are no structural problems which produce unfairness.
Sources of unfairness
A major source of unfairness is difficulty variation between questions or between rounds, though it is rare that this is pronounced enough for an entire tournament to be considered unfair. This is especially pronounced when there are systematic variations in the difficulty of a specific category or specific rounds, but is also true when it affects random questions. Because there are a finite number of questions in a round of quizbowl and a finite number of rounds in a tournament, teams which are affected by such variations will not be able to correct for the discrepancy. Because it is not possible to write questions which are all exactly the same difficulty, it is typically accepted that the questions in a set will take on some distribution.
The standard for a fair tossup is that it is well-written and pyramidal - this allows the maximum opportunity for a team with superior knowledge to buzz first and answer correctly. Conversely, an unfair question can be poorly written, apyramidal, or both. Things like factual errors, hoses, and swerves are inherently unfair, because they disadvantage teams which know more.
There is explicit unfairness in some bad quizbowl formats. College Bowl was infamous for playoff formats which disadvantaged particular teams. Single-elimination is often considered to be unfair, with double-elimination being the de facto standard for finals of sufficient size.
Before the standardization at 30 points, it was possible for different bonuses in a given round to be worth different amounts of points. A team could thus lose entirely because the bonuses they happened to receive were worth fewer points than the bonuses their opponent received.
Things that are not inherently unfair
Random chance is not unfair, though it may produce unexpected results.
There is significant debate with how fair existing rules for protests are, especially with regards to prompts. Existing rules typically award the team which won the protest with an uncontested tossup, which begs the question of whether that is an equivalent scenario to a player being prompted.