The ISR() is a macro to define a function to register with an interrupt vector.
The ISR() is defined in hardware/tools/avr/avr/include/avr/interrupt.h as below.
# define __INTR_ATTRS used, externally_visible
# define ISR(vector, ...) \
extern "C" void vector (void) __attribute__ ((signal,__INTR_ATTRS)) __VA_ARGS__; \
void vector (void)
The 'signal' in the __attribute__ means that it is an interupt handler. The 'used' tells the compiler to write code to a file unless it looks unused. The 'externally_visible' invalidates the effect of the '-fwhole_program', which is a gcc compiler option.