A noise canceling system is an active system that uses microphones to pick up ambient sound (noise), electronically inverts it, and then adds it to the wanted signal (out of phase) so the noise component is effectively canceled. This system can give you about 10 decibels of noise attenuation in addition to whatever you're getting from the isolation factor of the earbuds you're using.
The noise cancellation is higher at low frequencies, say 50-1.5kHz.
Sony makes noise canceling earbuds and I own a set, but they're a bit too big to fit under the helmet. I use the Etymotic ER-6i installed in custom molds instead.