Dr Alan Turing (1912-1954) was a theoretical mathematician and inventor. His parents lived in Guildford from 1927 and he visited there regularly throughout his life.
In 1936 Turing described the Turing Machine, a programmable computing device. Theoretical at the time, it was the prototype of all modern computers.
Turing worked as a system designer at Bletchley Park, notably designing the "bombe", an electro-mechanical device used to break messages encrypted by the German Enigma machine. Turing's work at Bletchley is thought to have shortened the Second World War by several years. Unfortunately this didn't become public until long after his conviction for homosexual acts and his death in 1954.
The atmosphere at Bletchley Park during the War was portrayed in Robert Harris' novel Enigma. The film of the book was produced by rock star Mick Jagger, who also funded a rebuild of Turing's "bombe" as a working prop. Jagger's bombe is now on display at the Bletchley Park Museum.
The 2014 film The Imitation Game claimed to be a biography of Turing. Sadly, it's a travesty of the real story.
Background notes for the talk can be downloaded here. These also contain links to some useful videos.
The speaker Simon Ritchie is a software engineer and his entire career has depended on the computer systems that Turing pioneered.
Turing did research in many fields in his short life and any talk about him will get into subjects that are technically difficult. The background notes and videos should help.