Most pistachios aren't that big of a challenge to open, but every once in a while you'll encounter one that simply will not crack under the pressure of nimble fingers. A nutcracker would be helpful, but if you don't have one of those, you can use a garlic press.
The above video can walk you through the process, but it's a pretty straightforward technique. Just put the stubborn nut in the press and apply firm pressure. The shell will pop off, and you'll have found a new use for a formerly single-use appliance.
How to Open a Tough Pistachio [America's Test Kitchen]