what is a Dyno tune

If you spray nitrous you should - at the least - get a handheld tuner to retard your engines timing. Dyno tunes help your car make max power all across the power band. The most useful thing they can do is play around with your A/F ratio. Most of our bolt-on's help the car either suck in or blow out air/gases at a faster rate. This can either result in a lean or a rich condition depending on what bolt ons you're running and how well you tuned the car yourself. A dyno tune isn't REALLY necessary but it's a nice piece of mind to know that your car is pulling extra HP in the safest possible manner. Running lean can blow your motor.

just basically having a professional look at live information being put off by your car and then tweaking it to your wants and needs such as nitrous