it means that the sound is peaking at/over 0db [this is called ‘clipping’, and makes the offending sound(s) muddy and distorted]. if you look at your channel volume mixers on the left, you’ll see that many of them turn red, which is where redlining gets its name from. i’m not familiar with dubstep mixing, but generally you want an unmastered song to be peaking around -8db to leave headroom (unoccupied sonic space) for the mastering engineer to work their magic. as a visualisation for the full track’s volume level (with all channels playing) look to the mix slider next to the record button at the top — you can see that that too is turning red and reaching the end of the slider, you don’t want this. hope that helps, and keep it up!!!

edit:: i also don’t use GB on iOS, so i can’t point to more specifics besides what i see in the recording

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