"Fire" is easily understood. "Brimstone" is a word used by some Bible translations to translate the Hebrew word of the Old Testament, pronounced gof-reeth, and the Greek word of the New Testament, pronounced thie-on. Both mean burning sulphur, or other molten natural elements (if you have ever been in an iron foundry, as this writer was for a summer job nearly 30 years ago while a high school student, you know the hot, acrid smell of "brimstone").
God's Warning To Those Who Refuse To Repent
"He condemned the cities of Sodom and Gomorrah by burning them to ashes, and made them an example of what is going to happen to the ungodly." (2 Peter 2:6 NIV)
"Surely the day is coming; it will burn like a furnace. All the arrogant and every evildoer will be stubble, and that day that is coming will set them on fire," says The Lord Almighty. "Not a root or a branch will be left to them. But for you who revere My Name, the sun of righteousness will rise with healing in its wings. And you will go out and leap like calves released from the stall. Then you will trample down the wicked; they will be ashes under the soles of your feet on the day when I do these things," says The Lord Almighty." (Malachi 4:1-3 NIV)
"And the beast [see The Lawless One] was taken, and with him the false prophet [see The Great False Prophet] that wrought miracles before him, with which he deceived them that had received the mark of the beast, and them that worshipped his image. These both were cast alive into a lake of fire burning with brimstone." (Revelation 19:20 KJV)
"Then He left the crowd and went into the house [see Capernaum]. His disciples came to Him and said, 'Explain to us the Parable Of The Weeds in the field.' [see Parables Of Jesus Christ] He answered, 'The one who sowed the good seed is the Son of Man. The field is the world, and the good seed stands for the sons of The Kingdom. The weeds are the sons of the evil one, and the enemy who sows them is the devil [see That Old Serpent]. The harvest is the end of the age, and the harvesters are angels. As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out His angels, and they will weed out of His Kingdom [see The Kingdom of God] everything that causes sin and all who do evil. They will throw them into the fiery furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. He who has ears, let him hear." (Matthew 13:36-43 NIV)
"And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever." (Revelation 20:10) (see Could Satan Repent?)
"And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them. And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works. And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death. And whosoever was not found written in the book of life was cast into the lake of fire." (Revelation 20:11-15 KJV)
Fact Finder: Will the entire earth one day be purified with fire when "a new heavens and a new earth" are made?
2 Peter 3:7,10-13
See also Planet On Fire!