Wikipedia
Lords-in-Waiting (female Baroness-in-Waiting) are peers who hold office in the Royal Household of the Sovereign of the United Kingdom.
There are two kinds of Lords-in-Waiting: political appointees by the Government of the day who serve as Government whips in the House of Lords; and non-political appointments by the Monarch, as an honour for retiring courtiers, who sit as crossbenchers.
As members of the Royal Household their duties are nominal, though they are occasionally required to meet visiting political and state leaders on visits to the United Kingdom. For instance, on 23 May 2011, when Barack Obama travelled to the United Kingdom from Ireland one day earlier than had originally been planned, he was greeted by the Lord-in-Waiting Viscount Brookeborough, who met him on behalf of the Queen. In 2014, Viscount Hood welcomed President Michael D. Higgins of Ireland to the UK. Again in 2015, Viscount Hood welcomed Chinese President Xi Jinping on a State Visit to the UK. As political appointees, the Sovereign appoints them on the recommendation of the Prime Minister.
A number of non-political Lords-in-Waiting are also appointed, as well as Permanent Lords-in-Waiting, who are usually retired senior officials of the Royal Household. These, being non-political, are at the personal discretion of the Sovereign. For example, Lord Janvrin who was, until 2007, the Private Secretary to the Queen, was installed upon his retirement as a life peer in the House of Lords and serves as one of Her Majesty's Permanent Lords-in-Waiting.