Exodus 16:36 - Ellicott's Commentary On The Whole Bible

Now an omer. — The “omer” and the “ephah” were both of them Egyptian measures. One — the latter — continued in use among the Hebrews, at any rate, until the captivity (Ezekiel 45, 46); the other — the omer — fell out of use very early. Hence this parenthetic verse, which is exegetical of the word “omer,” and may have been added by the completer of Deuteronomy, or by some later editor — perhaps Ezra.