What does this thingy do

mohammed
Www.twitter.com/el7ommed/status/22973291…
this is my nikon d3100…
there's this sort of a button or trigger on the left buttom side of the lens mount
can any one explain to me what does it do!
Added (1). T.co/d2oKgSME
heres a pic

G Wiz
Sounds like the lens release button.

fhotoace
If you look on page 2 of your user manual, you will see that it is the lens release button or are you asking about the Fn button or flash mode button?
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