What does speed +12 mean ?

It means that if you want him to have a better rate in normal instruction, you should increase speed of 13 points.

Demi-cerveau: It means that if you want him to have a better rate in normal instruction, you should increase speed of 13 points.

In French please

This means that you have to improve by 12 points if you want a better grade.

I think that it means that to have the most optimal stats (highest overall) for the given position and instruction you should increase the said stat (skill) by the said amount. If you improve the said stat by x points you might see then that the other stat "should be primarily improved" now. That's how I see it (although I completely ignore it).