Tenure, in the academic sense, is the formal process by which a scholar will become a lifetime professor at a college or university. Tenure-track positions are those offered to scholars who will work for an extended period of time doing research, teaching, going to conferences, and make the equivalent of pauper wages.
Some institutions will have multiple candidates in the same department working simultaneously for the same position, creating wondrous levels of stress and drama which would be considered psychological torture by The Hague. However, should the prospective scholar survive what truly could be Lord of the Flies with more tweed, the newly-tenured professor is given an insured position which he or she can only lose perhaps by murdering someone, and then he or she would be very politely asked for a resignation letter beforehand. This has led to hilarious embarrassments for notable institutions who have perfectly qualified scientists who conveniently also moonlight, under the protection of the First Amendment, as bloggers (e.g. Kevin MacDonald, Professor of Psychology, California State University - Long Beach, and delusional anti-Semite.)
Tenure is a hold-over from the founding traditions of academia when universities and churches were one and the same and professors were also priests. Modern Catholic universities do continue this to some degree, especially at institutes such as Providence College in Rhode Island,[1] which includes a large swathe of Dominican friars, sworn to celibacy, poverty, and obedience. In exchange for these vows, the Church assured the livelihood of the Professoriate who would then teach the larger population. In many Commencement ceremonies, often the pomp will include the academic chalice of the institute, yet another hold-over from the days when religion was the education racket.
In the modern era, the idea of Free Speech is symbolized for many by the concept of academic freedom, meaning that the professor will be protected despite the unpopularity of their opinions if their scholarship is demonstrably valid. In the era of World Wars, the anti-war views of professors were notable signs of protest, and the sacking of professors like Bertrand Russell were scandals. Noam Chomsky and Howard Zinn notably drove a burning hot spike into the side of Richard Nixon by protesting Vietnam while being protected by tenure, making a position on his "Enemies List."
But, as with everything else, there is an obvious gigantic gap for abuse to occur. The aforementioned MacDonald is one of the millions of idiots who do have a doctorate, but one absolutely unrelated to their oddball side interests. In this regard, combined with the fact that it would be very easy for a respectable professor to in fact be a sociopath, some see tenure as a structural enabler, something that welcomes abuses, and that it should be abolished (notably Harold Bloom[2]). But that would inherently abolish a major tenet of what is considered free speech. You're damned if you do or don't.