Skill trading, also known as "Week Training" (Swedish) or "Step Training", is the practice of buying a player and then sell him after just one pop. The purpose is to sell the player with one better skill (but otherwise the same as before) to a much higher price and quickly make a good profit.
You don't need to do this kind of trading in a serial manner. Sometimes you just buy a trainee for long term training, but when he pops immediately you decide that it is more economically advantageous to capitalize on selling it.
Some managers, known as "skill traders", specialize instead in predicting player's sub-levels and buying the ones that are very close to the next level. They try to make that player pop in just one training.