Instruct the compiler to generate a hex/bin file that starts at the specified address.
$ROMSTART = address
The address where the code must start. By default the first address is 0.
The $ROMSTART directive is an inheritance from BASCOM-8051. In the AVR it does not have any meaning.
In the 8051 you can use and relocate external memory. This is not possible in the AVR and hence there is no practical usage.
For a bootloader you can use the $LOADER directive.
The $ROMSTART directive is still in the help because a future AVR processor might make the option useful.
$ROMSTART = &H4000