Professional tutors teach to individual students. Not only is every lesson explained for the benefit of one single student, but the program is also individual.
A tutor knows what to teach each student because each student has an individual program. The tutor must evaluate the student's current levels and adapt the program to what the student knows and does not know. One way to check levels is with an assessment or series of tests. Most professional tutors will make a program based on assessments.
These assessments tell the tutor what to put into the program. What is the child missing according to her or his grade? These are called "learning gaps" or "gaps". When a student is behind in school, they did not miss everything. They do not need to go back to the previous grades. Students who are behind need someone to teach them what they missed, not to do it all again.
If a student does not go back and learn what she or he missed, the school will not go back. That is what tutoring is for. It is to do what the schools cannot do, to go back just for one student and teach them just what they need.
If a student does not go back to learn the "gaps", then the student will be missing some of the basics. Not knowing the basics makes each grade more difficult, and more gaps are created. The reason is that every year, the learning gets more difficult. Each grade has to build on the previous grade.
Professional tutors know this - they make sure the student fills those gaps to get to level.
Another aspect of professional tutors is that they know teaching methods to help each individual student. Tutoring is not giving the answers. Tutoring helps a student get to up to grade-level in the most direct way possible.