A workout training split is when you take your entire body and divide it up into different training sessions. Some training splits are designed with muscle building and weight gain in mind, others for cardiovascular endurance, and others for a specific sport. There are tons of ways to create your workout program’s split. Some very popular ones are “whole body splits”, in which you basically, as the name implies, train the entire body in that session. Another well known split is the “upper body / lower body” split, which you train in one session the muscles in the upper body, and in another session the muscles in the lower body.
Some also love the “push / pull” split, where you work out muscles that are “pushing” muscles, such as the chest, triceps, and shoulders, and at another training session you would train the muscles involved in “pulling”, as is the case with your lats, biceps, and forearms. And on and on. Now, which workout is best to perform will depend on what you’re aiming for. In this case, if you’re reading my site, you’re mainly after straight up hypertrophy, aka gaining weight, size, and muscle building. You’ll have to take into account that you can’t group together too many muscles in one session since that would make it hard to get in the amount of volume, sets, reps, and time that it takes to properly stimulate a particular body part to grow in size.