In terms of fungi, Epigeous means that the fruiting body fruits above the ground, think traditional mushrooms with mushrooms that grow above the ground.
The Prefix ‘Epi-‘ means ‘above’ and the suffix ‘geous’ relates to ‘geo’ meaning ‘earth’. So ‘Epi-geos’ is ‘Above the earth’.
Hypogeous means that the fruiting body fruits below the ground, this is mainly applicable to most Truffles that fruit below the ground.
The Prefix ‘Hypo-‘ means ‘under’, so ‘Hypo-geos’ is ‘under the earth’. Similar to the word ‘sub-terran’ which translates directly to ‘below the earth’.
This was some useful insight that I gained from Marcus Padilla Brown during his talk about Truffles at South West Funga Fest. I was able to use my background in etymology and linguistics to also add more etymological explanation to the above words. Hopefully this helps.