TCCR1A is timer/counter 1 control register A
TCCR1B is timer/counter 1 control register B
TCNT1 is timer/counter 1's counter value
CS12 is the 3rd clock select bit for timer/counter 1
TIMSK1 is timer/counter 1's interrupt mask register
TOIE1 is the timer/counter 1 overflow interrupt enable
So, the code enables timer/counter 1 at 62.5 kHz and sets the value to 34286. Then it enable the overflow interrupt so when it reaches 65535, it will trigger the interrupt function, most likely labelled as `ISR(timer0_overflow_vect)ISR(timer0_overflow_vect)