This plaque marks the spot where Lord Brooke, who was preparing to mount his assault on The Close, was killed by John 'Dumb' Dyott, who fired down from the top of the cathedral's central spire. The accuracy of his fatal shot was thought to be a sign of good fortune as the date was 2nd March - St Chad's Day. St Chad's being a local church that pre-dates the cathedral.
(Chad died on 2 March 672, and was buried at the Church of Saint Mary which later became part of the cathedral at Lichfield)
The set up is here