These are players that belong or is bundled with your soundcard program.
They simply allow it's user an offline playback/editing capability.
If your system does not have such a player fear not, the net has lots of FREE
players. When searching for a player, you can also look for an app that can
allow you to compile and play all your favorite midi files...these are called
MidiJukeBoxes.
Don't confuse the above players with browser plugins. Plugins are for 'served'
or 'streaming' Midi.

Midi is Musical Instruments Digital Interface.
It was formulated in the early 70's by keyboard manufacturers such as
Roland, Yamaha, Oberheim, Moog, and others.
What was deemed a celebration for this forum was due to the collective
reasoning and understanding by each participant to 'talk' via digital means among each keyboard/synthesizer maker. A simple 5-din cable allows the communication between those connected. It makes on instrument the 'master' and the other a 'slave'. In simple logic, 'master' plays his 'own' sounds or patches, while playing the 'sounds' of the 'slave' as a backdrop to the 'master'.
This configuration makes one sound 'fatter' as in thickness.
Imagine a grand piano sound with lush strings on the background. That 'was'
midi back then.

Nowadays, we incorporate and embrace that concept into something we can all use...a computer! not too many years ago during the early 80's did we find useful techniques using Midi and a computer...it's now known as 'sequencing'.
Many software programs allow the user to 'record' layers of different instruments much like multi-recording or 'overdubbing'.

This way the user can actually 'sound' like an orchestra or band without the added cost of 'hiring' individual musicians. A one-man-band concept was generated.