This setting is for advanced users.
Each Keycard has a pairing password which is being used as part of the process to set up a secure channel between the card and a host. This pairing password has a default value (see this article about Keycard core concepts if you want to understand better). It means that in most cases you will never have to change your Keycard pairing password. You can change it if you are a developer or if you want to use Keycard with a software wallet which does not use the default value.
To change your pairing password on a Keycard by using Shell, insert the Keycard in Shell, insert the PIN code and go to Settings > Keycard > Change pairing pass