What happens when -5v goes into an input pin?

An analog signal -5v to +5v. What happens when values below 0 enter the analog pin?

bad things happen. There are diodes that shunt it to GND but I think Atmel recommends not exceeding 1 mA of current for negative voltage inputs. Worst case it would blow the diode and then destroy that input pin.

Why are you asking this question ? This is NOT how the forum works. If you are even asking that question you are clueless inexperienced and need to state that at the beginning of your post and then explain what you want to do:

ie: "Hi , I'm clueless new when it comes to electronics and I want to....."

Then we take if from there.

Don't be rude, its a perfectly reasonable question for someone to ask on this forum.

Yes your arduino will be instantly destroyed if you put -5V on an input. Stick to a value between 0V and 5V (or 3.3V if its a 3.3V Arduino). Bipolar analog signals would normally be offset to a 2.5V centre voltage for input to an analog input. Lots of analog sensors do this for instance.

If we know what you were trying to do we might be able to suggest alternatives options.