We negotiate more frequently than we probably think. Not only at work, but also in our private life. With a sales person when buying something valuable like a car or a house. With our partner, children. The negotiation result strongly depends on the quality of preparation and BATNA is something we definitely want to know before starting any negotiation.
The acronym BATNA stands for Best Alternative To a Negotiated Agreement and represents your best 'no deal option'. Consider this example: You want to buy a car (specific brand and model). So you go to the car dealer of that brand and they offer you a price XX. You then try to get a better deal from another dealer. So when you start a price negotiation, your BATNA is XX. Sometimes you may have multiple 'no deal' alternatives. So you need to select the best one of them as your BATNA. Of course, in negotiations you never accept an offer worse than your BATNA. Knowing counterpart's BATNA is always a big advantage, so don't reveal your. One of dirty negotiation strategies may be pretending to have a much better BATNA than it really is. For example in a salary raise negotiation the employee may pretend already having a job offer paying the requested salary (thus indirectly threatening to leave if the salary raise request is not accepted). This may work but it is also possible that the Manager 'calls the bluff' and says "well..., we cannot give you that much, so let me wish you good luck in your new job". So never gamble with more than you can afford to lose ...or better always stick to ethical principles :)
Knowing your BATNA is one of main preconditions for a successful negotiation. If BATNA is just too bad, i.e. worse than any potential agreement, then we hardly can call it negotiation as it is either poker or dictate.