As Nephi walked home,
3 And it came to pass as he was thus pondering -- being much cast down because of the wickedness of the people of the Nephites, their secret works of darkness, and their murderings, and their plunderings, and all manner of iniquities -- And it came to pass as he was thus pondering in his heart, behold, a voice came unto him saying:
4 Blessed art thou, Nephi, for those things which thou hast done; for I have beheld how thou hast with unwearyingness declared the word, which I have given unto thee, unto this people. And thou hast not feared them, and hast not sought thine own life, but hast sought my will, and to keep my commandments.
he heard a voice that said:
5 And now, because thou hast done this with such unwearyingness, behold, I will bless thee forever; and I will make thee mighty in word and in deed, in faith and in works; yea, even that all things shall be done unto thee according to thy word, for thou shalt not ask that which is contrary to my will.
I'll bless you forever and make you mighty, because I know you'll never ask for something that is against my will.
So you'll get whatever you want.
6Behold, thou art Nephi, and I am God. Behold, I declare it unto thee in the presence of mine angels, that ye shall have power over this people, and shall smite the earth with famine, and with pestilence, and destruction, according to the wickedness of this people.
You are Nephi and I am God.
I give you the power to smite the earth with famine, disease, and destruction, because people are so wicked.
7Behold, I give unto you power, that whatsoever ye shall seal on earth shall be sealed in heaven; and whatsoever ye shall loose on earth shall be loosed in heaven; and thus shall ye have power among this people.
Whatever you seal on earth will be sealed in heaven, and whatever you bind on earth will be bound in heaven. 
You'll have power over everyone.
If you tell this temple to split into two pieces, it'll split in two pieces.
If you tell this mountain to become smooth, it will.
If you say that God will smite people, I'll smite them.
So go tell the people to repent, or I'll kill them all.
12 And behold, now it came to pass that when the Lord had spoken these words unto Nephi, he did stop and did not go unto his own house, but did return unto the multitudes who were scattered about upon the face of the land, and began to declare unto them the word of the Lord which had been spoken unto him, concerning their destruction if they did not repent.
After the voice stopped speaking, Nephi returned to the multitudes that were scattered about upon the face of the land.
He delivered God's message -- that unless they repent they'll be "smitten even unto destruction."
13 Now behold, notwithstanding that great miracle which Nephi had done in telling them concerning the death of the chief judge, they did harden their hearts and did not hearken unto the words of the Lord.
By this time, however, the people had forgotten all about Nephi's "great miracle" from the last chapter. 
15 And it came to pass that when Nephi had declared unto them the word, behold, they did still harden their hearts and would not hearken unto his words; therefore they did revile against him, and did seek to lay their hands upon him that they might cast him into prison.
So they hardened their hearts and tried to cast Nephi in prison.
17And it came to pass that thus he did go forth in the Spirit, from multitude to multitude, declaring the word of God, even until he had declared it unto them all, or sent it forth among all the people.
But God wouldn't let them do it.
Nephi was teleported away by the Spirit. 
The Spirit beamed Nephi from one place to another so that Nephi could deliver the message to all of the people pretty much at the same time.
After Nephi delivered God's message, the people were so diviced that they began to kill each another with the sword.