In AoA (Age of Aincrad) floors are the various sections of the game that the players must complete to clear the game (there will be 100 floors in total).
Consider each floor as a level; you start off on floor one (level one), and after raising your character's level decently you must go through the floor's dungeon to find the boss.
Upon defeating the floor one (level 1) boss you gain access to floor two (or level 2). There are 100 floors within Aincrad, and all floors must be passed to clear the game.
However, you can only kill a boss once. Once a player has killed a boss, they will no longer be able to fight that boss again or help someone else stuck on the floor boss in question.
Once a floor boss has been cleared, players do not have to kill the floor boss to progress to the next floor; it will be optional for the player at that point.
The players start the game on the first floor. To get to higher floors, the players must make their way through the dungeon to find the boss on their current floor, and defeat him. This will grant the player (and his/her party if a party applies) access to the next floor.
As stated above, Aincrad has 100 floors in total, and all floors must be passed to clear the game.
On every floor there are: