The Stop Button on the Tevo Tarantula and other 3D Printers is some what hidden behind the acrylic. Because of that you have to use a Pencil or something other small to push it. You can also use it as an "Emergency Stop" Button. Because the Stepper drivers release the motors if the Microcontroller is released.

Usefull if you change something on you machine or just want a easy to use stop button (:

Cheers!

PS: A short video about it: https://www.youtube.com/watch?v=pcNMHYDVOcI