NieR is an action role-playing video game developed by Cavia and published by Square Enix. The game was originally released only in Japan as NieR Replicant, but the series of games is now international, and hugely popular.
If you are new to the Nier games, maybe try NieR Automata first. Whilst technically the sequel to NieR Replicant, it builds upon the original game with faster/improved combat, a clearer storyline, and has alternate / dramatic playthroughs.
If you love NieR Automata, you may have more patience for NieR Replicant’s old-fashioned open world, slower pace gameplay. Playing NieR: Automata first may spoil some parts of NieR Replicant’s story but at least you will know what is going on :)