Ambient noise is the same thing as background noise. You hear these terms used frequently when discussing devices with built-in microphones, such as Bluetooth headsets, computer headsets and webcams. The more ambient noise there is, the more difficult it can be to hear the caller.
Many manufacturers like to claim that their microphones feature "noise-canceling" capabilities. What this means varies from company to company, and just slapping the label on the device doesn't necessarily make it so. As with all electronics, you should carefully research the device first, and pay attention to what users are saying about the mic's capabilities.