I2C or I2C is short for Inter-Integrated Circuit, a synchronous serial communication protocol developed by Phillips for communication between a fast Microcontroller and relatively slow peripherals (like Memory or Sensors) using just two wires. Hence, it is sometimes also known as TWI (Two Wire Interface).   It can support 112 devices and run over 1 meter in length with proper setup. 

You have a SDA (Data pin) and SCL (Clock Pin).   It allows many devices to only use 4 wires to connect.   You have VCC, Ground, Clock and Data pins.

The breadboard in front of the Arduino devices has two +/- power rails.  The one closest to the Arduino device uses blue and yellow wiring.  Blue = SCL (Clock)
Yellow = SDA (Data)

The top +/- power rail connects the 5V and Ground
Red = 5V VCC
Green: Ground

The Raspberry Pi T-Cobbler has 5V on left side and 3.3V on right side power rails.   The Sparkfun Bi-Directional Logic Level Converter allows 3.3V on left side for the Raspberry Pi and 5V on right side for Arduino devices.   It has the I2C Bus connecting through the component.    Raspberry Pi is the Master and Arduino devices are all slaves.   The Raspberry Pi is a Micro-computer that is more powerful than the Arduino devices and easier to program.  

Raspberry Pi OS with Kernel 5.11 from May 2021 is installed on the Raspberry Pi.  It is 64 bit.   I connected the I2C LCD screens to it in this picture.  

Raspberry Pi 5V

The 5v pins give direct access to the 5v supply coming from your mains adapter, less power than used by the Raspberry Pi itself. A Pi can be powered directly from these pins, and it can also power other 5v devices. When using these pins directly, be careful and check your voltages before making a connection because they bypass any safety features, such as the voltage regulator and fuse which are there to protect your Pi. Bypass these with a higher voltage and you could render your Pi inoperable.

Why have both Raspberry Pi and Arduino in I2C network?   Arduino can do analog sensors.   Raspberry Pi is a full microcomputer that can host LAMP components and your web site.  You can put your photo resistors on the Arduino and use I2C to transfer the results in near real time to display on your Raspberry Pi web site.   You can add a Turing pi and build clusters on Compute modules for your web site, kubernetes, and other fun projects. 

Here are the basic I2C Configuration files for all the Raspberry Pi and Arduino devices.  On the Raspberry Pi, you need to enable I2C in Raspi_config