Tenure is significant in the field of education because it allows teachers to have their jobs guaranteed. This practice, its critics say, allows teachers to keep their jobs even if they stop improving their performance and even if their performance actually declines.
Once a teacher has worked a given amount of time in a job, they receive tenure and can no longer (typically) be terminated unless they demonstrate gross incompetence or engage in some kind of misconduct. What this means is that teachers who have tenure do not really need to try to improve if they do not want to. All they need to do is to keep from showing gross incompetence and they can keep their jobs indefinitely.
Critics of tenure see this as one of the major problems with the public education system. They feel that teachers get complacent and no longer try hard. This, they argue, harms students. If teachers did not have tenure they would constantly be at risk of losing their jobs and would therefore work harder.
So, the significance of tenure is that it is a major source of controversy that, to critics of the public educational system, reduces teacher quality and harms students.