Handshaking
RS-232 was invented in a time of low speed modems and really awful voice grade phone lines. Ths signal names are only slightly suggestive of their function. There is another sublety. Signals are inputs or outputs depending on weather it is a DTE(Data Terminal Equipment) or a DCE(Data Communications Equipment). A straight through cable is used to connnect a DCE to a DTE.
Here is a brief glossary
DCD - Data Carrier Detect -- A modem signal saying there was activity on the line.
DSR - Data Set Ready -- A modem signal saying the the modem was able to send and receive data
DTR - Data Terminal Ready -- A signal to the modem saying there was a Data Terminal connected to it that was capable of sending and receiving.
The remaining two signals are used to manage a half-duplex line, where it is not possible to transmit and receive at the same time.
RTS - Request to Send -- This signal is aserted by a device that wants to send data, that is to control the transmit data line.
CTS - Clear to Send -- This signal answers RTS and grants the other device the ability to send.
Since most UARTs have FIFO's, are full duplex, and run quite comfotably at higher baud rates than RS-232 allows, hardware handshaking has fallen into general disuse. That doesn't even account for the popularity of software flow control using Xon/Xoff protocol.