Short answers, pulled from the story.
A Gentleman Usher is an officer of the Royal Household of the United Kingdom, historically among the most senior officials of the king's Chamber. Their role was first documented in the fifteenth century. Today an establishment of ten Lady and Gentleman Ushers attends royal events, drawn from retired military officers.
Under Henry VIII, Gentleman Ushers were divided into three classes: Gentleman Ushers of the Privy Chamber, Daily Waiters, and Quarter Waiters. They guarded the doors of the king's private rooms, accompanied the king in the gardens, oversaw meals, and inspected the hygiene of houses the monarch intended to visit. Under Henry VII, at least one usher was required to keep the door from 8am until nightfall every day.
Sarah Clarke was appointed the first Lady Usher of the Black Rod on the 17th of November 2017. She formally took on the full duties of the role in February 2018.
The Gentleman Usher of the Black Rod was established around 1361 as an officer of the Order of the Garter. The role also encompasses duties as secretary to the Lord Great Chamberlain, Doorkeeper of the House of Lords, and, since 1971, Serjeant-at-Arms of the House of Lords.
There are six coloured rods: the Black Rod (established c.1361, Order of the Garter), the White Rod (c.1373, formerly for the Parliament of Scotland), the Green Rod (1714, Order of the Thistle), the Scarlet Rod (1725, Order of the Bath), the Blue Rod (1882/1911, Order of St Michael and St George), and the Purple Rod (1918, Order of the British Empire). Each rod designates an usher for a specific order of knighthood or institution.
Lady and Gentleman Ushers are generally appointed from retired military officers, with two representing the Royal Navy, four representing the Army, and four representing the Royal Air Force. Ushers retire at seventy and may then become Extra Lady or Gentleman Ushers.