As you know, you can have multiple devices on the I2C bus. So how would you identify the device that you want to read/write data to? That's why you need to define I2C_SLAVE_ID. You will find it in your device datasheet. The same with I2C_SLAVE_ADDR - what address are you going to write data to? Or read data from? You decide that - again looking in your device datasheet.
Hope that it helps.