On-Stage  3.5mm socket to 1/4in plug headphone adapter

Gold finish.

On one end it’s a small 3.5mm (1/8in) stereo socket.
On the other end it’s a 6.34mm (1/4in) stereo jack plug (aka phone / standard instrument / guitar plug).

Without getting too technical you will need this to connect your ‘ear buds’ into that bigger headphone socket found on things like:
A vintage hi-fi
The Marshall MS2 / MS4 mini amps
Most headphone sockets from at least 10 years ago
‘Professional’ recording / mixing desks and equipment
Modern High end hi-fi