it's when the dentist gives you counseling or advice on something. Most insurances will not cover this so be careful not to ask any questions when you are at the dentist or else you may end up getting billed for it. All you have to do is ask a simple question like "what brand of tooth paste do you recommend" and they will use that as a chance to counsel you and bill you. Of course, they don't tell you that they are going to bill you for it because they assume you already know that you'll be billed for asking questions.
I only ask questions because I have known my dentist for years and know that the doesn't try to rip people off like that.
Oral Hygiene Instruction billing tends to be when you go to a place like American Dental Center or any other dentist clinic that advertises with coupons and radio ad. They are a business created to make money... they will try to bill you for everything to help make profits and bonuses.