"It" is the Screaming Bell and all units in the Brigade. Skaven Brigades are not limited to just 4 units so this can be a pretty large number of units. It does not just mean the Screaming Bell and Infantry in the brigade either, it means ALL units in the Brigade, There must be at least 1 Infantry unit though (unlikely to be a problem) if the Bell is to move.
Uhrm.... I beg to differ......
IT = The BELL
the Bell can not move on initiative
the Bell is not driven back by shooting
by NO means the entire brigade is effected by this!.
MIND that a Bell, in contact with skaven infantry, could be declared excempt of being in the brigade, allowing the rest of the troops a move on initiative. But that simply means it stays in place (which often provides just what you need)