(There are some very vague, very technical, very Objective™ spoilers for Undertale below. You probably won’t get the joke if you haven’t beaten Undertale at least once anyway.)
Undertale is a 122.52 MB single player game with “partial” controller support described as an “RPG Game”. The game is rendered at 640×480 pixels like a 30 year old CRT television though almost all ingame art is actually made at a resolution well below 480p. Most enemies are black and white in combat with extremely limited animation. The color pallette is similarly limited, probably 32 colors or less on most screens. The game only uses three buttons and arrow keys.
The game has three endings. Two endings can be achieved at level 1, while the other ending requires more time grinding than playing the game. The “hardest” boss in the entire game has only 1 HP and their attacks can only deal 1 HP in return. The strongest equipment in the game is literally useless. Several fights cannot be lost.