The season ended, the injuries were fixed and the athlete is ready to begin a period of training again (goal specific, remember there are workouts and then there is training). Base training is exactly what one might think of when they look at a pyramid, the longest part at the bottom. It is what all other fitness traits are built upon. For example:
You cannot have speed without building up the cardio system.
You cannot lift maximum weight until you stabilize all the joints associated with the exercise.
You cannot run until you walk. Ok, maybe that one was too easy but you are getting my point now.
There has been a term coined GPP, or General Physical Preparedness. It is basically all the basics. For you english teachers out there, dont hate me for that sentence. Its getting the body ready to move together without specificity. It is training all the energy systems at the basic level, preparing the body for more detail and goal specific training later on.
Its the foundation of the fitness house you are building. Things included in GPP are basics like push ups, pull ups, sled dragging, tire ironing, washing clothes, walking, giving the dog a bath etc.
No really it is any non specific movement that makes the body work all energy systems together at a low enough rate that you build upon the progressive resistance you have applied. Long runs instead of sprints. You get the picture.
Next, the build phase.