Adolescence refers to the time between puppyhood and sexual/social maturity. This can be between 6 months and 3 years depending on the size and breed of dog.
For male dogs testosterone production can increase dramatically, resulting in difficult times for both the dog and owner. Likewise, females may act differently around the time of their first season. It can be difficult for them to concentrate and focus on what you are asking.
Castration or spaying is often suggested at this point to 'calm' the behaviour. This is often not that case and in some situations can make things worse, either way it is not simple quick fix.
We find that, more often than not, issues that were not a problem with a small, lightweight puppy, become a major issue as a dog reaches adolescence and is more confident. They can become impulsive, develop selective hearing and generally become more interested in their surroundings, and other dogs.