Jesus made use of the word 'Gehenna' ('hell) eleven times. Now Gehenna was a physical place - the Valley of the Son of Hinnom south-east of Jerusalem. (Gai Ben Hinnom in Hebrew)
In his time, it was the local 'rubbish' dump, a place of 'vile' things, animal carcases and the bodies of executed criminals. Permanent fires burned there (according to some stories, with the addition of brimstone) to prevent outbreaks of disease.
But this same Valley of Hinnom had a more sinister past...
Hideous practices had been carried out there in the name of Moloch (the old Canaanite Sun-God, Baal). The worship of Moloch involved sacrificing children by making them "pass through the fire". For a graphic description of this place check Jeremiah 19:26.
Fire, human sacrifice, and idolatry eventually became synonomous with Gehenna. Jesus used the word Gehenna as a symbol to illustrate the 'spiritual' horrors accompanying 'spiritual' isolation from the one, true God of Israel.
Put simply: 'hell' was a place where one was spiritually isolated from God; 'heaven' was its opposite.