# a tenth

This is one part out of ten equal parts. 

# a tenth of an ephah

"a tenth of an ephah (which is about 2 liters)" or "2 liters" 

# a grain offering of jealousy

"a grain offering for jealousy"

# a reminder of the iniquity

A "reminder" is something that shows evidence that something had occurred that required justice. In this case, he made the offering to determine whether his wife had committed adultery or not.