Your dog's ability to perform a known behavior in new places, with new people, and in new situations, rather than only in the exact spot where they learned it. Dogs are very context-specific learners, so a sit that is rock solid in your kitchen may fall apart at the park.

To help your dog generalize, practice each behavior in as many different locations and contexts as you can, starting easy and building up. Vary where you stand, the distractions around you, and who gives the cue. Each new setting is its own small challenge, so lower your expectations at first and let your dog succeed before you raise the bar again.

This is the whole idea behind proofing. A behavior is not truly reliable until your dog can do it anywhere, not just at home.