the short answer
An upper lower split is a training plan that divides workouts into upper-body days and lower-body days and alternates between them - most commonly four sessions a week, two upper and two lower. That structure trains each muscle about twice weekly, which tends to outperform once-a-week training when total volume is equal. It is a flexible default for intermediate clients chasing muscle and strength.
No split is magic. The real driver of growth is weekly training volume - the hard sets a client does per muscle - matched to what they can recover from and stick to. The upper lower split is simply one of the cleanest ways to organize that volume.