Their are two major Radio-Computer connection channels, CAT and Audio...
The CAT channel allows you to tell the radio what frequency and mode to tune to, which VFO to use and when to transmit and receive. This allows MacLoggerDX running on your computer to automatically tune your radio to a dxcluster spot it has just received. CAT also allows the computer to read back this information so that MacLoggerDX can automatically enter the frequency and mode you are using when you log a call.
Since most radios have old style TTL or RS-232 connections for this channel and most computers have abondoned these old style serial ports, we need to add a USB-to-Serial adapter to the computer. This is usually a simple piece of hardware accompanied by a software driver that allows MacLoggerDX to "see" the radio connected to this device. The device may also include level converters to convert the RS-232 to the TTL level required by the (older) radio and sometimes DTR and RTS lines to also let the computer key the radio's PTT and CW lines.
Some devices combine the above capabilities with digital CW generators and or modems for the Audio channel...
The audio chanel allows the computer to send pre-recorded speech or digitally encoded information (RTTY, PSK etc.) to the radio's microphone or accessory jack as well as receive digitally encoded data from the radio's headphone or acccesory jack for decoding.