Players earn Player EXP by training. After earning a certain amount of EXP their Player Level will increase. Players earn attribute points each time their level increases.
When a player levels up, he receives additional attribute points for you to distribute. If unsure which attributes to strengthen, tap 'Strenghten [AUTO]' for reference. Some attributes are more useful for forwards, others more for defenders, etc. The suggestion you will see fits the position your player is specialized for.
* Don't forget that the formation you choose and the position you put your players in have a huge impact on the strength of each individual player.