What does this Mosfet Symbol mean (two diodes facing each other at Vgs)?
It is an ESD clamp, showing that the gate is ESD protected. Typically the back to back diodes are shown as Zeners. This scheme protects against positive and negative ESD hits.


In the event of a positive ESD event, the top diode is forward biased, and the bottom diode's reverse breakdown voltage will be exceeded. The ESD is shunted to the source, which is often ground.
In the event of a negative ESD event, the top diode's reverse breakdown voltage is exceeded, and the bottom diode is forward biased. The ESD is again shunted to the source.