Foundation behaviors are skills that enable your dog to learn agility-specific behaviors.
Having a strong foundation enables you to train the things you envision as agility-related (obstacles, response to handling cues, coping skills for competition environments etc.) efficiently and effectively and will help you bring out the best in your puppy.
Having a weak foundation will limit the options that you have for training agility-specific behaviors which may ultimately impact how well your dog learns them and how well they can perform during competition.
The list of foundation behaviors is comprehensive and takes time to work through. Fortunately building these behaviors starts at home, in short play/training sessions. Many variations on these games can achieve the same goals. The videos and tips on this site will explain what foundation skills are valuable, why, and will demonstrate methods you can use to build them.
Building a strong foundation is one of the best things you, as a handler can do, to prepare your dog for competition and actually for real life as well.