The $100 charge which showed up on my credit card account, listed as HERTZ RAC FIREFLY, turned out to be a gas station hold for when I bought gas for my rental car. They hold $100 for gasoline while your charge is processed. Some gas stations do this, although I haven't seen it for a long time.
When you sign the rental car agreement, you give permission for this type of charge to be posted automatically to the credit card used to rent the car. Also parking violations. And moving violations. Read the small print.