The abbreviation HDMI stands for High-Definition Multimedia Interface, a technology used for transmitting uncompressed video data and compressed or uncompressed digital audio data to a computer monitor, a digital audio device or television compatible with the protocol from a HDMI device. To be able to transmit the data between the two types of devices, a cable is needed. The role of the cable is to transmit video and the audio signal without loss or with as little loss as possible.
The HDMI cables used for connecting the two different types of devices can be connected only one way – the plugs on each end of the cable fit only one way. In case one of the devices does not have a suitable output, a HDMI adapter will be needed – the small adapter attaches to the VGA or DVI connection on one device (the older one, probably) to allow for the cable to be connected. Once plugged, the cable makes the connection without requiring the user to install any driver on any of the devices connected and ensures the transmission of the desired signals between the devices. However, most adapters support only video signals, so if you need to transmit audio signals, too, you will probably need to find a different solution.
You can find more information regarding hdmi connectivity and cable functions at https://iec.net/.