This might help better understand... The following was taken from notes of someone in my church from lessons given by our pastor who taught us from the Greek.
2 Pet 2:4
4~~ For if - and it is true -
The God did not spare the angels that sinned,
but incarcerated/cast them down to Tartarus {tartaroo},
and delivered them into 'pits of gloom'/ 'chains of darkness' . . .
to be constantly guarded with reference/unto judgment. (Mat 25:41)
{Note: Tartaroo is the Greek word for one section of 'hell' in the center of the Earth. There are three sections of hell.
1. Paradise - where Old Testament Saints stayed before the
ascension of Christ. They are now all in heaven and this
Paradise is empty. There is another garden area in heaven
called 'paradise' today.
2. Torments - this is where all unbelievers go until Judgment Day
at the Great White Throne of Christ. After judgment, prophecy
tells us all will be condemned and thrown into the Lake of Fire.
3. Tartarus - Tartarus is the prison section where the fallen
angels who left their second heaven home and procreated
with women in Genesis 6 were thrown in captivity until
Judgment day. They are still there today.
grace and peace...