Which one ??
This is your Y homing switch.
you really need to know X-Y-Z. it WILL come in handy just like knowing right from left.
Just trying to say study this and pay attention.
Knowing which is the X, Y and Z axis is basic stuff. You should do a bit more study before you actually turn the machine on, I think. This forum is a gold mine of information, and you should be using it.