"He did not understand you."
Translation:Il ne t'a pas compris.
Without any context, we have to assume that the person "he" is talking to is a woman.
The agreement of the past participate with verb "avoir" is very tricky: it agrees with the direct object IF AND ONLY the object is placed before the verb.
"Il n'a pas compris ses paroles" -> "compris" is invariable because object "paroles" is after the verb.
"Les paroles qu'il n'a pas comprises" -> "comprises" agrees with object "paroles" (feminine plural) because it is placed before the verb.