Read it HERE.
Summary: Jesus announces that the law of Moses is fulfilled in him—The Nephites are the other sheep of whom he spake in Jerusalem—Because of iniquity the Lord’s people in Jerusalem do not know of the scattered sheep of Israel. [A.D. 34]
Jesus Christ tells the Nephites some important things in this chapter: of note was that the Law of Moses has been fulfilled by Him.
Verse 2: "And it came to pass that when Jesus had said these words he perceived that there were some among them who marveled, and wondered what he would concerning the law of Moses; for they understood not the saying that old things had passed away, and that all things had become new."
Some of the people were a little confused. However, their hearts were ready to accept new doctrine, so Christ went on to explain that He fulfilled the law, but that the covenant was not entirely fulfilled (vs. 8). The institute manual does a good job of explaining what this means. It is a little lengthy but full of good information and clarification:
"Jesus said that "old things had passed away and that all things had become new” (3 Nephi 15:3). Elder Jeffrey R. Holland explained: “It is crucial to understand that the law of Moses was overlaid upon, and thereby included, many basic parts of the gospel of Jesus Christ, which had existed before it. It was never intended to be something apart or separated from, and certainly not something antagonistic to, the gospel of Jesus Christ. . . . Its purpose was never to have been different from the higher law. Both were to bring people to Christ” (Christ and the New Covenant, 147). Thus Jesus could say, “For behold, the covenant which I have made with my people is not all fulfilled; but the law which was given unto Moses hath an end in me” (3 Nephi 15:8).
"What did Jesus mean when He said, “The covenant which I have made with my people is not all fulfilled”? (3 Nephi 15:8). Jehovah made a covenant with Abraham anciently. Abraham was promised (1) eternal posterity, (2) a land that would eventually be the celestial kingdom, and (3) God’s priesthood power. These promises were also made to Abraham’s descendants (see D&C 132:30–31) and will be fulfilled in the future.
Finally, another powerful part of this chapter was Christ's explanation of the people in Jerusalem. He said that they were too hard hearted, so He could only tell them that He had other people. He said that because of their iniquity, they did not know of the people in the Americas. He also says this:
20 And verily, I say unto you again that the other tribes hath the Father separated from them; and it is because of their iniquity that they know not of them.
There are people all over the world, and Heavenly Father is mindful of each one of them. My last thought from the institute manual is especially powerful. It discusses Christ's analogy of being a shepherd to His sheep--or his people.
"One day a missionary, meeting a shepherd on one of the wildest parts of the Lebanon, asked him various questions about his sheep, and among others if he counted them every night. On answering that he did not, he was asked how he knew if they were all there or not. His reply
was, ‘Master, if you were to put a cloth over my eyes, and bring me any sheep and only let me put my hands on its face, I could tell in a moment if it was mine or not’” (George M. Mackie, Bible Manners and Customs [n.d.], 33, 35).
Jesus Christ does know us. He cares for us, guides us, comforts us. We just need to trust Him and follow Him in return, and we will always be fed.