A more thorough explanation:

A letter of recredentials is a formal letter that is sent from the diplomatic secretary of state of a host country to a minister or ambassador who has been recalled by their own country. The purpose of this letter is to officially accredit the foreign minister back to their home country.

When the ambassador of Country A is recalled by their government, they must return home. However, if they are later sent back to Country B, the diplomatic secretary of state of Country B will send a letter of recredentials to officially accredit the ambassador back to their post in Country B.

Another example could be if a minister from Country C is sent to attend a conference in Country D. Once the conference is over, the diplomatic secretary of state of Country D will send a letter of recredentials to officially accredit the minister back to their home country.

These examples illustrate how a letter of recredentials is used to officially accredit a foreign minister or ambassador back to their home country after being recalled or sent on a mission to another country.