The abyss is identified as the same place that held the "angel of the abyss" mentioned at the fifth trumpet (Rev 9:11).

It is described as bottomless pit (Rev 9:1), a furnace with lots of smoke (Rev 9:2), and a prison for fallen angels that is strong enough to hold them and requiring a key (Rev 9:1; 20:1-3).

Elsewhere in the Bible, the abyss is presented as a place of confinement and punishment for fallen angels who indulged in some form of gross immoral behavior (Luke 8:26-31; Matt 8:28-32; Jude 1:6-7).

From Revelation: An Explication © Helpmewithbiblestudy.org: A resource for learning how to read the Bible.