An action noun is a word that performs the function of a noun (a subject or an object) but describes an action. They are often formed by taking a verb and adding a suffix (often -ion, -ness, or -ing):

verb --> action noun
explode --> explosion
reverse --> reversal
inspect --> inspection
believe --> belief
smoke --> smoking
arrive --> arrival
forgive --> forgiveness

In SC, these can show up in parallelism. Ex: OG13/2015 #121:
"New theories propose that catastrophic impacts of asteroids and comets may have caused reversals in the Earth's magnetic field, the onset of ice ages, the splitting apart of continents 80 million years ago, and great volcanic eruptions."

It's rare that the GMAT would test you on a difference between an action noun and a verb. (I can't think of an example off of the top of my head - maybe another expert can remember one?). If you saw such a difference, though, it would likely come down to meaning, or the structure of the rest of the sentence (e.g. if it was in a parallel structure).