Hi,

I need help understanding this schematic and what the zener diode does here.

Thank you

 

It's a TVS diode to protect the circuit.
Read the data sheet.

 

If the voltage goes over the 6.4V, it will conduct and blow the fuse. (it might start conducting a bit before 6.4V)
It basically protects the circuit from any overvoltage.

 

It's called a "crowbar".

 

It's a clamp not a crowbar. It maintains the voltage at 6.4 volts. It will only blow the fuse if the transient is large enough to draw over 500ma through the diode.

 

Another protection against polarity reversal is supplying the wrong polarity of the supply voltage. In this case, the fusible link will burn out, but the protected device will not fail.