In a "conventional reservoir", natural gas (which is lighter than water) migrates upwards through a porous (hole-filled) rock until a barrier stops that migration. That barrier can be structural (think of it as an underground hill), or it can be stratigraphic (the holes in the rock stop abruptly & the gas can no longer migrate). These barriers thus form what are known as "traps".

Geologists spend much time & money in the quest for subtle conventional gas "traps".

This good porosity allows the gas to easily migrate into the wellbore so that it can be produced & sold at relatively high rates.

The term "unconventional reservoir" is given to rock which has so few holes (so little porosity) that the gas never migrates & thus stays in place. Given the low porosity, unconventional gas wells are typically less prolific than conventional reservoir wells. However, if enough acreage can be assembled to warrant testing & infrastructure, many wells can be drilled across an unconventional play, thus making for an economic venture.