Yes 'pop means a skill has imcreased a full level ie gone from Respectable to Strong. You still getting a bit confused by Stamina and Game Shape. Stamina affects how long a player can play at a level; before they start getting tired. Game Shape affects how well they play, so they are different. Most people advise never training Game Shape because it can be controlled by good minutes management. Training Stamina once or twice a season is probably enough.