what is the meaning of atr=atr(Periods)?

I'm new to pine script, trying to understand the follwoing code:

    //@version=4
study("Supertrend", overlay = true, format=format.price, precision=2, resolution="")
...
Periods = input(title="ATR Period", type=input.integer, defval=10)
...
atr= atr(Periods)
...

What is atr? variable? function? api function?