Simplify the following text for a Grade 2 native English-speaking person.
Original: Speed mods can increase or decrease the speed the arrows travel up the screen ( sometimes making the arrows easier to read ) . Modifiers can also make the arrows more difficult to read , by adding effects such as the arrows only appearing when they reach halfway through the screen ( or only appearing until halfway ) , hiding the arrows or the step zone altogether , making them rapidly accelerate as they reach the arrows , or travel up the screen in a `` wave '' .
Simplified: The arrows are in time with the music .