Pretty obvious to me - 'First, that we have granted to God, and by this present charter have confirmed for us and our heirs in perpetuity, that the English Church shall be free, and shall have its rights undiminished, and its liberties unimpaired.'
In other words, King John and his successors, get back in your box until Henry VIII comes along.
If you are interested in 1215, there's a great book with that title by Danny Danziger and John Gillingham - I'm re-reading it at the moment.