What does :"The often-used phrase 'pay attention' is apt: mean? could you explain?
"The often-used phrase 'pay attention' is apt: you dispose of a limited budget of attention that you can allocate to activities, and if you try to go beyond your budget, you will fail. It is the mark of effortful activities, that they interfere with each other, which is why it is difficult or impossible to conduct several at once.

Question

What does :"The often-used phrase 'pay attention' is apt:  mean? could you explain?
"The often-used phrase 'pay attention' is apt: you dispose of a limited budget of attention that you can allocate to activities, and if you try to go beyond your budget, you will fail. It is the mark of effortful activities, that they interfere with each other, which is why it is difficult or impossible to conduct several at once.

Answer

apt:  appropriate or suitable    http://learnersdictionary.com/definition/apt

The author is saying that the phrase "pay" attention is appropriate because he is comparing one's attention span to the restrictions of a limited budget.  In other words, attention is a finite resource just as money is.