The "push/pull/legs" split is one of the most common ways to organize a training week, and it's built directly on how your major muscle groups actually work together — not an arbitrary division.
Push muscles are the ones that extend a limb away from the body: the chest (pectorals), shoulders (deltoids), and triceps. A bench press, overhead press, and dip all rely on this same group working together, which is why they're trained on the same day — hitting one usually pre-fatigues the others anyway.
Pull muscles do the opposite motion, drawing a limb or the torso inward: the back (lats and traps), biceps, and rear deltoids. A row, pull-up, and bicep curl all draw on this group. Because push and pull muscles are largely antagonist pairs — one flexes a joint the other extends — balancing volume between them is part of what keeps shoulder and elbow joints healthy over time.
Legs cover everything below the hips plus the core that stabilizes them: quadriceps (front of thigh), hamstrings (back of thigh), glutes, and calves, along with the abdominals and lower back that brace the spine during heavy lifts like squats and deadlifts. Legs make up roughly half the body's total muscle mass, which is why leg day tends to demand the most recovery of the three.
Understanding which exercises overlap in muscle group is more useful than memorizing a list of "chest exercises" or "back exercises" — it's what lets you build a week that trains everything without any one muscle group getting hit on back-to-back days.