The number of training points you have shows you how far you have progressed in your current training level.
You need a certain number of training points to level up each training level. When you gain a level, you can unlock a new drill or upgrade a previously unlocked one.
Each drill description indicates how many training points will be earned per player. For example, if you train 10 players with a drill that gives +5 training EXP per player, you will earn 50 training points in total.
Tap the level icon on the Training top screen to view information about how many training points you need to level up.