An onboarding buddy is someone who can help you adjust to cultural norms and direct your questions as a "catch-all." In addition to answering basic technical questions around workflow or code, the onboarding buddy is effectively lending their social capital to you to get you started. They are the safety net when you don't know who else to turn to.
Rahul recommends that the onboarding buddy and manager be two different people. Your manager relationship contains a power dynamic in that you're interacting your boss, so you may not be fully vulnerable with them. The onboarding buddy is closer to a peer, which means you can get the help you need.
One of the most important roles for the onboarding buddy is to help you figure out your support channels.