Here's how this icon on HDMI plug looks:
Note, that this plug is on a DisplayPort→HDMI cable.
I found the same icon online on a VGA plug:
2 Answers
What does a rectangle with two lines on the sides symbol mean?
Display symbols
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A rectangle with two lines on the left and right side is a symbol used to represent the VGA connector on many computers and devices.
Source What are all the symbols used by computers?
4After seeing DavidPostill's answer, I investigated further.
On a DisplayPort→HDMI cable, this icon indicates the cable's end, which is intended for the receiver of the signal, e.g. a monitor, a projector, or a TV. This cable can be used to connect a DisplayPort of a GPU to an HDMI monitor, but it can not be used to connect an HDMI output from a GPU to a DisplayPort monitor. It is an important quirk of HDMI compatibility with DisplayPort:
... Although the dual-mode standard specifies a method for DisplayPort sources to output DVI/HDMI signals using simple passive adapters, there is no counterpart standard to give DisplayPort displays the ability to receive DVI/HDMI input signals through passive adapters. As a result, DisplayPort displays can only receive native DisplayPort signals; any DVI or HDMI input signals must be converted to the DisplayPort format with an active conversion device. DVI and HDMI sources cannot be connected to DisplayPort displays using passive adapters.
