Basic IO drivers such as UART, I2C etc. for Atmega8/16...
Add F_CPU = xx in Project Properties → Toolchain → AVR/GNU C Complier → Symbols where xx is the frequency of the μC.
I2C driver enables user to use TWI interface to communicate with other μC or sensors having I2C interface.
UART driver enables user to use TWI interface to communicate with other μC or communication modules such as Bluetooth, Wi-Fi modules, etc.