This is something that you probably don’t need for your Eurorack synth: An input grounding plug.
The purpose of this grounding plug is to override the “normal” connection of an input jack, forcing the input to 0 volts (ground). It’s simply a 1/8" plug with the signal (tip contact) connected to ground (sleeve contact), and a ground symbol painted on the side so I don’t forget what it is.
In my homebrew modular system I use the CV and gate lines on the Eurorack system bus to play notes. These lines are driven from a keyboard or DAW, and I have several modules with gate or volt-per-octave pitch inputs which are normalled to the system bus. This saves a lot of cabling in the type of patches I usually make. (Most Eurorack modules don’t take advantage of this feature, which is one reason why you probably don’t need this plug.)
However, there are times when I want a module to not use the volt-per-octave pitch signal — for example, using a VCF as a fixed-frequency formant filter — so I need it to ignore the normalled signal from the bus. The usual way to do this is to plug a patch cable into the input and leave the other end of the cable hanging. That disconnects the normalled signal and leaves the input completely unconnected.
In theory, you’re not supposed to leave an input hanging like that because it could pick up noise. Hence, this grounding plug, which makes sure the unused input has a solid ground connection while being disconnected from the normalled signal. (In practice, I haven’t noticed any problem with unconnected Eurorack inputs, which is another reason why you don’t need this thing.)
As a bonus, this plug looks better than a dangling cable. Maybe that’s why you need it.