Why can a president get reelected only once

Can someone read this article and tell me the 1) key ideas and 2) one key quote (copy and paste the link the article is not to long at all)from

In 1990, the average income for an american worker was $1,518 per month. in 2000, the average income for an american worker was $2,632 per month. during this time period, a basket of necessities (food, housing, etc.) increased from $657 per month in 1990 to $1,402 per month in 2000. the real value of income ▼ increased decreased over that 10-year period.

If both the president and the vice president can no longer serve, who will become the next head of state?