A screen on four wires

A character display, not a screen

The glass shows sixteen characters across and two down. Every cell is five dots wide and eight tall, and the shapes come out of a character table baked into the controller. You send it the byte 0x41 and it draws an A. There is no way to light an arbitrary dot, so there is no way to draw a logo, a graph or a curve.

What you can do is define eight characters of your own, a cell at a time, and print them alongside the built-in ones. That is enough for a degree sign, an arrow, or a bar that fills a dot column at a time — eight characters of your own is that chapter.

Sixteen pins, and why you never see them

The row of holes along the top is the display's real interface, and it has not changed since the Hitachi HD44780 defined it in 1987. Driving it directly costs six wires even in its economical four-bit mode, plus a supply, a ground, two more for the backlight, and a potentiometer you have to supply yourself for the contrast.

The chip on the back of this board does all of that on the board. It is an I²C port expander: two wires in, eight outputs out, and those eight outputs are wired to the six signals the controller needs plus the backlight.

What it costs you

Two signal wires, and they are the two you were going to spend anyway. SDA and SCK are a bus — every I²C part on the bench shares the same pair, so a sensor, a real-time clock and this display together still cost two pins.

The bill is paid somewhere else instead. The display is a 5 V part, and it puts 5 V on those two shared wires whenever nothing is talking. On an Arduino Uno that is exactly what the Uno expects. On an ESP32, a Pico or a Raspberry Pi it is more than the pins are rated for, and why it wants five volts is the chapter that decides whether your board survives the project.