A high side driver is used for driving a load whose ground is floating (It isn't connected to circuit ground ). For example, to drive the two MOSFET's in the upper part of a H-Bridge circuit you need a high side driver, since their source terminals aren't always connected to the circuit ground and therefore at varying potential.
On the other hand, a low side driver can only drive loads whose ground is always at the same potential with the circuit ground ( Or the voltage difference between these grounds is constant).
Regards.