The plain average of the last n closes.
Every candle in the window counts the same. That makes an SMA calmer than an EMA of the same length, and slower to react.
The 50- and 200-day SMA are watched by many traders, which is exactly why price sometimes reacts around them.
Worked out by the same code the builder uses, on stored daily candles. Move a setting and the line moves with it. This is what happened, not what will happen.
Gate: Price close is above SMA(200) [1d].
When an old, large candle drops out of the window the SMA can jump, even though nothing happened today.
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