What the heck is a client?
A client is an individual user, usually a personal trainer's customer. If you are using this application for personal use, client management doesn't impact you.
Fitness professionals add their clients to the system by creating new users. New users will be automatically assigned to the trainer that added the user. At this point, you can "manage" the client; update their body attributes and goals, create workouts to build an exercise program, and track their progress.
Gyms and wellness professionals can also create users. They can then assign trainers to their users, who in turn can manage the individual's data.