# 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.