If you have only played the "basic game" then you have only seen roughly 30% of what Nier: Automata has to offer.

"A" Refers to the first play-through, the one you play as 2B.

"B" Refers to the second play-through, where you play as 9S. 9S sees and learns things 2B didn't, showing you the world of the game in a new light. Essentially, there's a lot more story that happened during the time 9S and 2B were separated. He also has different combat mechanics to 2B.

"C" Refers to the third "play-through", which is actually just a straight up sequel to the main game that continues the story past where it ends in A and B. In C you play as 2B, 9S, and a bit as A2.

"D" and "E" refer to the two different endings you get depending on who you side with at the end of C. Getting these five endings leads to the true final ending, where you fight the credits with the help of other people who have received this ending. Achieving this ending allows you to sacrifice your save and become one of the people helping new players achieve the super-difficult final true ending. The game will actually, for real, delete everything, if you choose to do this, but it is widely considered part of 100%ing the game.

The remaining endings "F" through "Z" are joke endings, the ones where the credits roll past really fast and you reload a save, because you found some dumb way to die. Like eating a mackerel or uninstalling your OS chip.

If you dont know what MetalEdge means, you have probably only ending A. The weird thing about that game is, ending B is more or less the first part of the game again, just with 9S instead of 2B. But there is more story after that with ending C and D.