Ninja uses the Alienware 25 Gaming Monitor with 1080p resolution and up to 240Hz refresh rate, which goes for around $480. It comes with Nvidia's G-Sync technology that helps prevent stuttering and screen tearing — where parts of the screen looks like it's lagging behind other parts.
The number next to the "Hz" — or hertz — is how many times the screen can refresh itself every second. The higher the number, the more frames per second it can display. And the more frames per second, the smoother the gaming experience.
In Ninja's case, 125 frames per second is the goal, which is on the high-end of smooth gaming.
What's a little surprising is that Ninja is using a 1080p resolution monitor. It's fine, but his graphics card can easily handle the higher 1440p resolution, which offers sharper details. In my experience, sharper monitors make enemy players easier to spot and makes for more accurate aiming, too.
Either way, Ninja uses what Ninja likes.