The first time Sheol is mentioned in the Bible is in Genesis 37:35. There we read,
“For I will go down into the grave unto my son mourning”
These words came from the mouth of Jacob, who was lamenting the (he thought) death of his son, Joseph. In this context, the word “grave” is actually Sheol.
There are many descriptions of Sheol, but the best one is in Luke 16. Christ told the story of the rich man and Lazarus. They both died and both went to Hades. The rich man lifted his eyes and saw Lazarus who was with Abraham. They could not come closer because there was a “gulf” between them; but they were able to see and hear each other. Both were conscious; Lazarus was comforted, but the rich man was being tormented and was able to remember his family.
That was the work that Christ accomplished by dying on the cross and resurrecting. He delivered His saints out of Sheol and brought them to Paradise, to be with Him forever.