The No Child Left Behind law was a major piece of President George W. Bush's pre-9/11 agenda. He believed that it was important to improve the educational standards and achievement levels of American public schools.
The NCLB's main thrust was to require frequent testing of students to ascertain whether they were improving their skills in core subjects. The goal of the law was to have all students in American public schools be performing at grade level in core subjects by 2014. The law also required that each school make progress towards this goal each year between the law's passage and 2014. They were supposed to show progress in every subgroup (racial groups, special education students, etc) every year. The law provided for sanctions against schools that did not make enough progress every year.
The law is very controversial both on the left (because of its emphasis on testing and its punitive aspects) and on the right (because it gives a great deal of power over education to the federal government.