At the time of the appearance of the resurrected Jesus Christ to the Nephites, the multitudes went forth, felt the resurrected body of the Savior, and testified that "it was he, of whom it was written by the prophets, that should come" and then cried out "with one accord, saying: Hosanna!" (3 Nephi 11:15-16).
According to Daniel Ludlow, the word "Hosanna" is a transliteration of a Hebrew word of supplication which means in essence "Oh, grant salvation." Evidently the people were asking the Savior to teach them the way to salvation; thus it is not surprising that He immediately teaches them the basic principles and ordinances of the gospel. [Daniel H. Ludlow, A Companion to Your Study of the Book Of Mormon, p. 261]
According to McConkie, Millet, and Top, the word "Hosanna" literally means "Save now, save we pray" and is taken from a messianic psalm found in the Old Testament (see Psalm 118:25). It was commonly used in ancient times in connection with their worship of Jehovah at the Feast of Tabernacles. Shouting Hosannas and waving palm branches was a means of worshipping the Messiah and acknowledging his saving power (see Matthew 21:9, 15). . . . In the modern Church also the "Hosanna shout" is used as a sacred means of worshipping the Lord and expressing our profound respect, love, and gratitude for him and his holy mission. The modern proclamations of "Hosanna" are usually reserved for deeply sacred events such as temple dedications. Whether done anciently or today, it is a symbol of deep reverence for and worship of our Lord. [Joseph F. McConkie, Robert L. Millet, Brent L. Top, "Doctrinal Commentary on the Book of Mormon, Vol. IV., p. 53-54]