The word שְׁאֹל (She’ol) is found 66 times in the Massoretic Text1. It is a dark (Job 17,13) and silent (Psalm 115,17) place with gates (Job 38,17; Isiah 38,10) where all humans go (Psalm 49,10-11). Being there is like being in a deep water under a mountain (Jonah 2,2-7): it is a world under the world. So it is an "underworld" (which is no more under the earth than heaven is above the sky): it is a spiritual place. Proverb 15,11 tells us that:
Sheol and Abaddon lie open before the Lord — how much more, human hearts
’Avaddon means something like "the world of those who disappeared". It is synonymous2 with She’ol. It is like if the part of us that goes in She’ol is our heart (לֵב - lev) which refers to the inner part of man
In old times this place was considered as a land of oblivion/forgetfulness (Psalm 88,12) where no one praises the Lord (Psalm 6,6; 88,11; 115,17) nor put his hope in him (Isiah 38,18). The dead, it was thought, live there an attenuated form of life as signifies Ecclesiastes 9,10:
there is no work, planning, knowledge, or wisdom in Sheol where you are going.
But it doesn’t mean that God is not there any more (Psalm 139,8). God is the master in She’ol (Isaiah 7,11; Job 26,6): so if one goes there without God's permission, God will drive him out (Amos 9,2), and if he wants he sends some down to She’ol, and he raises others up (1Samuel 2,6). So what Ecclesiastes 9,10 implies is that it will be too late to do anything to choose our final destination even if it is said that if from She’ol no one returns (Job 7,9), God can rescue the faithful from it (Psalm 30,3-4) because His power is superior to the power of She’ol (Psalm 49,15; Hoseah 13,14).
1 The Hebrew Bible as we have it today.
2 It seems that אֲבַדּוֹן (’Avaddon) expresses the same idea as שְׁאֹל (She’ol) with the added idea of ruin or destruction.