Universal Asynchronous Receiver/Transmitter (UART), a serial port, is a communication interface where the data is transmitted serially in bits from a sensor to the host computer. Using a serial port is one of the oldest forms of communication protocol. It is used in data logging where microcontrollers collect data from sensors and transmit the data via a serial port. There are also sensors that transmit data via serial communication as responses to incoming commands.
We will not go into the theory behind serial port communications (there's plenty of theory available on the Web at https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter). We will be discussing the use of the serial port to interface different sensors with the Raspberry Pi.