There are lots of sites that explain it with words well, already. So we're going to try a slightly different approach. Let's take a typical school day. The teacher instruct the students, both verbally and by writing it on the board, what the assignment is, and they are to start working on it. Child is redirected.
Child tries to do assigned work, but drops pencil on the floor.
Again the teacher realizes the child has lost concentration. Again, patiently, the teacher calls the child back to the task at hand. Child goes back to work and is doing well. Teacher now calls the reading group over. After the group finishes reading, the teacher instructs them to go back and finish their seat work.