In simple terms, serial data is sending data over few wires in a timed sequence. This is the opposite of parallel in which data may simultaneously be transmitted over many wires.
To take advantage of the benefits of serial communications, standards have been devised:
First was RS-232, a standard that has evolved over time and used as a basis for the TTL Serial interface, lower level connectivity between two electronics boards.
Later was the Universal Serial Bus (USB), devised to be better than RS-232. With refinements and operating system innovations like "Plug and Play", USB has dominated modern electronics device connectivity.