flxad41 wrote:Adafruit_SSD1306 display(OLED_MOSI, OLED_CLK, OLED_DC, OLED_RESET, OLED_CS);
That line declares an instance of the Adafruit_SSD1306 class named 'display'.
flxad41 wrote:void Adafruit_SSD1306::display(void)
That line defines a member function named 'display' of the Adafruit_SSD1306 class
In the sketch, you could call display.display(), which would call the 'display' method of the class instance 'display'. It is kind of confusing, isn't it?