The "Second Invitation" refers to the current period of time when the LDS Church has recently been "reborn", much like the natural man must be "born again" to enter the Kingdom of God. The "First Invitation" began with Joseph Smith restoring the fulness of the gospel through the Book of Mormon and then founding the Church of Christ in 1830.
After his martyrdom, the various splinter churches have had varying degrees of success, but none has preserved all of the gospel truths, hierarchy, and ordinances correctly. Thus, it was necessary for the Angel Moroni to return again with the gold Plates of Mormon and the interpreters and give them to a new prophet in Brazil named Mauricio Berger. He translated the first few of them into The Sealed Book of Mormon, published in March 2019. Then the Church was officially "reborn", with the new headquarters in Brazil, and a branch here in Duchesne, Utah.
The teachings of the Sealed Book, and new revelations to the seer Mauricio, along with other LDS scriptures, constitute the foundation of the Second Invitation to all to be rebaptized and accept even more of the gospel of Jesus Christ.