layering is exactly what it sounds like. superimposing different sounds to achieve texture and different sound. for example, you can layer drums to make it bang harder. you can filter out a drum sound and layer it with the original so you get more lower end freq. or you can take a kick drum sound with sharp attack and layer over a boomy kick to make it deep. possibilities are endless. just experiment with what sounds good to you. its 4 30 in the morning, i need to goto sleep
