The grave is also known as Hades.
There is more to that verse:
Revelations 20
12 And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.
13 And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done.
14 Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.
15 And if anyone's name was not found written in the book of life, he was thrown into the lake of fire.
It is supported by these:
Rev 21
8 But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”
Rev 2
11 He who has an ear, let him hear what the Spirit says to the churches. The one who conquers will not be hurt by the second death.’
They're not meant to be places. They are depicted as people or demons or angels, or something else.
Revelations 6
7 When the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, “Come!”
8 I looked, and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him.
They were given power over a fourth of the earth to kill by sword, famine and plague, and by the wild beasts of the earth.