The forward reference refers to the case when we point an address space of a smaller data type with a pointer of a bigger data type This can be pictured as allocating memory in single bytes and accessing it with integer pointer as chunks of 4.
ISR refers to the Interrupt Service Routines. These are procedures stored at specific memory addresses which are called when certain type of interrupt occurs. The ISRs cannot return a value and they cannot be passed any parameters.
The unsigned int is typecast into the signed value.