In Geometry Dash, you can use Alpha Triggers as a way to set the Opacity of any object.
Opacity refers to the transparency or visibility of any object (ie. how “see-through” any object is). An opacity setting of 0.00 makes any object entirely invisible, and 1.00 makes any object fully visible. Alternatively, you can enable the Hide option in Edit Group if you're absolutely certain that you want an object to stay invisible.
Fade Time refers to the time it takes for any object to fade into the set opacity, in seconds.
Whether or not you want a level to fade to black, or if you want to make an object invisible then reappear, the alpha trigger is the best trigger for the job.