This means you'll need to back off the signal you're sending into GarageBand from your electric guitar and/or pedalboard. If you're too loud in GarageBand, you'll see the red clipping indicator in the top right corner where the master volume is located.
This is something I have to watch with my compressor and EQ pedals, since they both have the ability to boost my signal. This means that the pedalboard is in a kind of preamp role, with the potential to send enough gain into GarageBand to create unwanted noise or even distortion. With your pedalboard going directly into the USB interface, and ultimately GarageBand, you'll need to treat GarageBand as your final power amp. The less pedals you use, the less noise you'll have to deal with. So, nothing new here - just common sense.
