The experiment has a fixed number of trials. For instance, a negative binomial distribution may read as "Out of 40 trials...," meaning that there are a fixed number, 40, of trials in this experiment.
Each trial can only have two possible outcomes. In the common illustration of a coin toss, the only two possible outcomes are heads and tails. In general, the outcomes are referred to as "successes" or "failures." For instance, if you are tossing coins and wanting the the probability of getting exactly 30 heads out of 40 trials, then heads are the successes.
The probability of success or failure remains the same for every trial. For instance, a typical coin toss will have a 50 percent probability of heads and 50 percent probability of tails for every toss. A weighted coin may have an 80 percent probability of heads and a 20 percent probability of tails for every toss.
Every trial is independent and does not have any effect on any other trial. For instance, one toss of a coin does not affect the outcome of the next coin toss or any coin toss after it.
The negative binomial property says that the experiment is repeated until the predetermined number of successes is achieved. For instance, if the experiment is looking for 23 tails, then the coin is tossed until 23 tails have turned up.