Translation:I do not understand a word; speak slowly.
I believe that «ни» followed by any genitive singular word is going to be somewhat of a set expression which means "not a _," or "any ___(plural)." To express that you do not understand a certain word in particular could probably be accomplished with the following Russian sentence:
«Я не понимаю слово.»
The «ни» here emphasises that the speaker does not understand any of the words that are being spoken.