"Functional Training" is the concept of training players for the functions that they must perform in their positions on the field. Functional training becomes more and more important as players enter the U15 age groups and above, because one way that great players differentiate themselves from others is by being better than others at executing the key functions of their position. Functional training can be either technical or tactical.
Technical Functional Training is training that focuses on the techniques that are most important (or most commonly used) in specific positions. For example, technical functional training for forwards would involve different types of finishing, and functional training for outside backs may involve learning to strike an angled, driven ball.
The session below shows technical functional training for defenders in the technique of clearing balls out of the air. The activity involves both some basic agility training as well as the actual technical execution.
Tactical Functional Training is training that focuses on the decisions that are most commonly made in specific positions, and the cues that help players differentiate between good and bad decisions. For example, tactical functional training for outside midfielders may involve understanding when to cut inside the field on the dribble to go to goal vs. when to serve early into the box vs. when to attack the endline.
The session below shows tactical functional training of how the defensive back four and defensive midfielders can find space to build an attack out of the back (instead of just serving the ball forward optimistically). This is a later stage of the session, where players form different teams are going "through" each other in the building progression. (This creates passive opposition that makes the decisions more realistic but still highly successful.)