Geometry Dash The Nightmare is a rhythm game where you have to feel the music and get the square to the finish line without hitting any obstacles. This is the second game in Demon Pack 3, one of the levels with Easy Demon difficulty.
Your cube will always automatically move forward at a constant speed, so it is inevitable that you will encounter obstacles. Jump over them, fly over them, dodge them, do everything you can to avoid hitting them because you don't want to have to go back to the starting line.
Although it is considered difficult, there are still players who believe that there are Easy Demon levels that are more difficult and can do better. Regardless, this is a popular level and is well received by the global Geometry Dash community.