Basically, RS-232 terminals are the hardware devices that contains a keyboard and a display, communicate using a serial interface, one bit at a time.
These terminals uses a 9 and 25-pin connector where one pin is for transmitting the data one is for receiving the data, and one is ground.
The other pins are used for various control functions, but most of which are not used.
Serial lines are lines in which characters are sent one bit at a time.
All the modems uses this interface.
Now, to send a character over the serial line to an RS-232 terminal or modem, then the computer system must transmit it one bit at a time, prefixed by a start bit, and followed by 1/2 stop bits just to delimit the character.
To allow a remote user to communicate with mainframe in the mainframe computer world, RS-232 terminals are still commonly used.
Even when they are replaced by PCs, the personal computers often simply emulate the old RS-232 terminals to avoid having to change the mainframe software.