"I saw in heaven another great and marvelous sign: seven angels with the seven last plagues - last, because with them God's wrath is completed." NIV translation
John sees seven (the number of completeness) angels (messengers) from God who are to deliver God's complete wrath to the earth. We may ask ourselves: What's so "great and marvelous" about this sign? The simple fact of the matter is that all of creation has groaned under the weight of sin and its effects. This sign tells us of the coming end to that curse and the renewal of all things.
"And I saw what looked like a sea of glass mixed with fire and, standing beside the sea, those who had been victorious over the beast and his image and over the number of his name. They held harps given them by God" NIV translation
Now, John sees, in this sign, the victorious saints. They are victorious because they stood firm in their faith in Jesus Christ even to the point of death. To those who watched them be killed it probably seemed like they had been defeated but, as we see, their victory is being in the presence of God forever.
Revelation 15:3 & 4
"and sang the song of Moses the servant of God and the song of the Lamb: 'Great and marvelous are your deeds, Lord God Almighty. Just and true are your ways, King of the ages. Who will not fear you, O Lord, and bring glory to your name? For you alone are holy. All nations will come and worship before you, for your righteous acts have been revealed.'" NIV translation
Those that have been killed during this time for not taking the mark of the beast are now singing praises to God in heaven. They sing at the wonder of the power of God and are glad for His judgment which is right. After all that they have seen and been through, they are celebrating that it is over and that they are now with the Father. It will be plain to everyone on earth and in heaven that God is right in judging the earth and its inhabitants. All nations will bow to the living God.
"After this I looked and in heaven the temple, that is, the tabernacle of the Testimony, was opened." NIV translation
The "tabernacle of the Testimony" is talking about the Ark of the Covenant that was made in Exodus 25. This was the designated place where God would meet with the high priest and signified His presence.
"Out of the temple came the seven angels with the seven plagues. They were dressed in clean, shining linen and wore golden sashes around their chests." NIV translation
Then, John sees seven angels coming from the presence of God. They were dressed in white linen (a symbol of purity) and wore golden sashes (a sign of royal authority). Angels are messengers from God and these messengers carry God's wrath for those that have defied Him until the end.
"Then one of the four living creatures gave to the seven angels seven golden bowls filled with the wrath of God, who lives for ever and ever." NIV translation
The living creature that John saw here is one of the creatures described in Ezekiel 1 and they are kind of like a royal honor guard for the Father. Here, one of them distributes the bowls containing the complete wrath of God.
"And the temple was filled with smoke from the glory of God and from his power, and no one could enter the temple until the seven plagues of the seven angels were completed." NIV translation
Where there is smoke there is fire. God is a consuming fire and at this point nobody can come into His presence. God's amazing power is about to be demonstrated on the earth.