From Halopedia, the Halo wiki
Chirurgeons were dedicated to unlocking the Lifeworker secrets locked within the Forerunner keyship, Anodyne Spirit, simply known to the Covenant as "the Dreadnought". Staffs of them were stationed aboard specialized DSC-class agricultural support ships fitted with arcane instrumentation. The chirurgeons on these ships and those on High Charity were responsible for the creation of Prelates, biologically-enhanced San'shyuum used in specific combat roles.
Chirurgeon is an archaic term for surgeon.