I don't know why so many people find it so difficult to comprehend this concept. "if the weapons in the game its fair no matter what." my arse.
This is about balancing. Any weapon that is so effective that every other weapon pales in comparison is unbalanced. Ideally, different weapons should have strengths in certain areas, weaknesses in others, meaning that the right combination of skill and weaponry = win. If every weapon has as many weaknesses as strengths, the mark of a good player is learning how to play to minimize the effects of their weapon's weaknesses while magnifying its strengths. For example, a submachine gun is only moderately accurate and has low stopping power, but has a high rate of fire and allows maximum mobility, therefore it suits close quarters combat but leaves its user vulnerable in other situations. A sniper rifle is accurate and powerful but limits mobility, has a small magazine capacity, a low rate of fire, and in many games is also horribly inaccurate when fired without using the scope as well. In both of these cases a skillful player learns to move and behave in such a way to take full advantage of their weapon's excellence in one area, while avoiding situations that would put them at a disadvantage. So good snipers don't run around in close quarters combat, and good submachine gunners don't take potshots at the enemy from a distance.
An unbalanced weapon would be something that has significantly more advantages than any other weapon. For example, an assault rifle with the magazine capacity and rate of fire of a high capacity SMG like the P90, and the damage and range of a sniper rifle, would be significantly overpowered and would be effective in more or less any situation, throwing the whole skill angle of managing your weapon's drawbacks and advantages out the window. Therefore, it could be considered a 'noob weapon', because less skill is required to utilize it effectively. Makes sense?
Of course, most of the time when people brand something a 'noob weapon', it really isn't unbalanced, they're just bitter about having lost. That doesn't make the concept irrelevant.