The bit_is_set() is a macro that checks if the second argumen-th bit of the first argument is 1.

The bit_is_set() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.

#define bit_is_set(sfr, bit) (_SFR_BYTE(sfr) & _BV(bit))

It calculates the logical AND of _SFR_BYTE(str) and _BV(bit).