The video poker machines that say 100% payback with optimum pay, what do they mean by optimum play?

First thing you need to realize is video poker machines DO NOT play the cards like they were using a real deck.    I have won far more Royal Flushes than I have Straight Flushes.   That is because they are set to pay back a percentage of the money that goes into them and since Royals get more excitement and notice in the casino,  you get far more of them than you would normally playing poker with cards.    To counter that the pay frequency on the lesser hands is lower,  you do not hit a a straight flush,  straight or flush as often as you would in a real game.   So what they are saying is if you play perfect you will get 100% of your money back.   Big deal.   What you have to do to get ahead is to realize the odds are against the middle hands and go for the royal at every opportunity.   That means if you have three of the cards you go for it.   Even if you have to give up something less. 

The max you can bet over a LONG period of time. Yeah, you WILL win, but you may have to play it for weeks to win. 

It means making the correct decision every time you have to make a decision. 

The correct decision is the one that gives you, on average, the maximum return.

If you can choose which game to play you have to pick the game with the best payout percentage.

If usually means playing the maximum number of credits since that usually gives you a higher percentage payout for a royal flush.

It means making the best decision when you decide which cards to replace. For example assume you are originally dealt

The king of hearts

The queen of hearts

The jack of diamonds

The ten of hearts

The nine of hearts

in a game that pays when you have a pair of jacks or better.

depending on the payouts for each final hand the optimal choice might be

(1) to throw away no cards and get paid for a straight.

(2) to throw away the the jack of diamonds

(3) to throw away the jack of diamonds and the nine of hearts or

(4) to throw away the ten of hearts and the nine of diamonds.