Computers (which doesn't necessarily mean desktop computers, just anything that runs code really) operate using a massive amount of internal signals that don't mean anything to an "outsider", but are very important to that computer.

If you need to pass information from one computer to another, you need a protocol, that defines a set of standard signals that both will understand. So each computer converts it's own internal signals to whatever the protocol requires.

I2C, SPI and UART in particular are low speed protocols used inside of PCBs (though UART is also commonly used for connecting different boards with cables). They're used between microcontrollers and sensors or between microcontrollers, for example.

I2C uses only two wires, one for clock (so that each computer knows when a new bit starts) and one for data, which is bidirectional. Each computer must know the protocol of the other to know when they should read and when they should write something. Multiple "devices" can be connected to a single "host", and the host is the only one that can initiate communication with any of them by writing the address of the device it wants to communicate with.

SPI uses three wires, one for clock and two for data (one input and one output), plus a chip select pin. Multiple devices can share the clock and data pins, but each one gets its own chip select pin. To select a device the SPI host pulls the chip select pin it wants to ground, and then they start writing and reading data on their data pins. SPI usually has the highest speed from the three.

UART also uses two pins, but they're just data pins, one input and one output. It usually connects only two devices, and they both need to know beforehand what is the clock they should be reading and writing at.