The lightning bolt over the battery icon in the menu bar indicates the AC adapter is charging the battery or batteries installed in your PowerBook. If you have the Control Strip installed, you will also notice the lightning bolt in the Battery Monitor portion.
Figure 1. Control Strip Charging Indication
Figure 2. Menu Bar Indication
When the battery is fully charged, the lightning bolt will disappear from the menu bar battery icon. The Control Strip Battery Monitor portion will indicate the presence of the AC adapter by showing a small power plug through the battery as shown below.
Figure 3. Control Strip AC Adapter Indication