On most modern phones, you can even unlock devices with your fingerprint. (howtogeek.com)
Its specs aren't all on par with most modern phones due in part to HTC's decision to go with a previous - generation Snapdragon 600 processor, but performance is still quite smooth in most cases. (bgr.com)
The short answer: it's a Subscriber Identity Module, a small circuit board in most modern phones to communicate with your carrier. (imore.com)