Tenure is a system within academia that provides job security for professors while ensuring their freedom of expression and research. It is an agreement ensuring that a professor cannot be fired from an institution without 'just cause', meaning they cannot be removed for voicing a particular opinion or pursuing particular research.
Tenure track positions are coveted among professors, as they offer a kind of job security that is uncommon among most professions. Though tenured professors must continue to meet professional benchmarks and can lose their status over serious matters such as misconduct, they are able to advance their careers with an enviable level of stability. This is one of the most attractive things about the career of a full-time professor.