A more thorough explanation:

Definition: Letters close, also known as letters secret, refer to a governmental document that is issued to a private person, sealed, and not made available for public inspection.

Example: In medieval England, the king would issue letters close to his trusted advisors, which contained confidential information about the state of the kingdom. These letters were sealed and not meant to be seen by anyone else.

The example illustrates how letters close were used as a means of communication between the government and private individuals. The letters were sealed to ensure that the information contained within them remained confidential and was not accessible to the public.