the short answer
A push pull legs split (PPL) organizes training into three days grouped by movement: push (chest, shoulders, triceps), pull (back, biceps), and legs (the whole lower body). It runs as a 3-day week, training each muscle once, or a 6-day week, training each muscle twice. It suits clients training 3 to 6 days a week who want a clear, repeatable structure - the 3-day version is the entry, time-crunched option.
The split is a way to deliver weekly training volume - it is not what builds the muscle on its own. The honest version: total weekly volume and progressive overload drive growth, and the best split is whichever one your client recovers from and keeps doing.