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The title says it all: What does this Mosfet Symbol mean (two diodes facing each other at Vgs)? I know its an N-MOS, but I don't understand the meaning/purpose of the two diodes.
Part: AO7404
Datasheet Link: http://www.aosmd.com/res/data_sheets/AO7404.pdf
Picture of the symbol (highlighted part):
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.
Looks little confusing . After looking that the datasheet is from a Chinese supplier one can guess it. edit: That the zener symbol is not correct. 
For anyone else looking for more information on this topic, this type of component is often called a transient-voltage-suppresion (TVS) diode and helps reject ESD as @Matt Young described.
Another example of its use can be found in the schematic of the u-blox C94-M8P rev B evaluation board. Note that the TVS diodes are protecting header pins that can be touched by the user.
