An upper/lower workout split is a training style that breaks your workout sessions down into two categories: Upper body workout days and lower body workout days.
On upper body workout days, you will train the muscles of the upper body. This will include the chest, shoulders, back, biceps, triceps, traps, forearms, and possibly core.
On lower body workout days, you’ll focus on the muscles of the lower body. This will include the lower back, quads, hamstrings, glutes, calves, and possibly core.