Quoting Rabbi R.C. Klein’s Word column:
The Vilna Gaon (to Proverbs 2:2-3, 2:6) differentiates between binah and tevunah by explaining that tevunah refers to the “reflection” that qualifies one’s chochmah or binah. The Vilna Gaon in Chemdah Genuzah (to Proverbs 1:1) writes that binah refers to understanding something on one’s own terms, while tevunah refers to understanding something so thoroughly that one can explain it to others (see also Zohar, Vayakhel 201a). Rabbi Shlomo Brevda (1931-2013) points out in Leil Shimurim (p. 26) that this latter source runs counter to the aphorism often cited in the “Yeshiva World” in the name of Rabbi Chaim Soloveitchik of Brisk (1853-1918): “A deficiency in being able to explain something is a sign of a deficiency in one’s actual understanding.”