If you have an external modem, those flashing or steady lights tell you what's happening in the ongoing "handshaking" or interaction between a computer (with its Universal Asynchronous Receiver/Transmitter or UART microchip) and the modem. Or, more formally, to use the RS-232C serial standard terms, between the Data Terminal Equipment (DTE) interface and the Data Communication Equipment (DCE) interface.
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|AA (auto answer)||The modem is ready to respond to an incoming call (assuming a communication program is also ready to handle the call). Since this light can be off even when the modem is ready, it's not useful.|
|CD (carrier detect)||This simply indicates that the computer and the modem have recognized each other and that a carrier connection is established. If a second call is received, carrier detect may be temporarily interrupted and, depending on a number of factors, the first connection may be dropped or lost.|
|HS (high speed)||If on, indicates that the modem is ready to transfer data at its highest speed. Since this is often the case, this light is also not very useful.|
|MR (modem ready)||If on, indicates that your modem is ready to operate.|
|OH (off hook)||If on, tells you that the phone line is ready for use.|
|RD (receive data)||When flashing on, indicates that the modem is receiving data or signals from a remote computer.|
|SD (send data)||When flashing on, indicates that the modem is sending data or signals to another computer.|
|TR (terminal ready)||When on, indicates that your computer's communications program is active. If it's not on, either the program or your computer may not be working.|