This word is commonly used to decribe some one with lesser knowlage of a noob. This is far worse than a noob, so in fact that they either dont even know how to spawn in a game, any of the keys and dont even know how to speak.
You are in game, There is a 'AFK', You go to knife the 'AFK' They then start to move on the axis's not strafing, this would be an example of a jabra,
Alternatavely, a Jabra maybe an experianced player, but is ignorant and not really a team player and will not comply to any commands or speak at all. So in a way jabra's are the downfall of society, they are unreasonable and no one likes them.