This is a very simple node which, given up to 9 input textures, let you select the one with the number given as last input. Be sure to give an integer number as input (obviously from 1 to 9)!
This can be useful if you want to create a driver to a custom property, to select one of the (up to) 9 input textures for the material.
In order to use the node in your .blend file, append the node from the downloaded texture_selector.blend, go to node editor, press Shift+A, then Group. If you correctly appended the texture selector, you'll find the node in the list.
Note that the performance impact of the node scales with the number and the resolution of the textures. For instance, connecting 9 4K textures might seriously slow down the loading time and the memory consumption of the material. In that case, it might be better to use different materials instead!
Finally, in the file you will also find a simple example.