Have you heard of the 3 D's of dog training? It's a pretty popular term that many trainers use. The 3 D's refer to Distraction, Duration, and Distance. These are one way to categorize the difficulties or challenges in dog training.
Distraction refers to both the environment and things in the environment that are distracting. Generalization (moving to a new environment to help your dog learn the behavior more thoroughly in all settings) is a form of distraction.
Duration is how long the dog is performing the behavior. It can also refer to the number of reps of a behavior, or even how many behaviors are chained together at one time.
Distance can mean a couple of different things. Distance between the dog and the handler is one part. The other is distance from a distraction(s). The farther the dog is away from a distraction, the easier it will be for the dog to perform behaviors.
If you think of Distraction, Duration, and Distance as challenges, it's easy to remember that you should only increase one at a time. However in practice, people often try to increase the challenges too quickly, or increase two or even all of them at the same time. In order for your dog to be successful, you want to be sure that two out of three are as close to baseline as possible.