SigmaSpikes is an indicator created by Adam H Grimes. It's a volatility indicator which applies a standard deviation measure on candles for a set period of time, in order to find big candles/moves relative to the other candles. These big moves could be the outcome of setups being traded by market players, large market orders put in by big money players, &/or HFT algorithms reacting to events (usually fundamental events).
These big moves can also be seen as inefficient as it doesn't fit in with the mostly efficient market - this is very similar to gaps of which price would want to fill as they're inefficient, in order to "restore order" to the market.
This indicator attempts to give better information at a glance, by highlighting the background of candles that have sigma spikes over the set standard deviation threshold.
In the chart snapshot image above featuring EURUSD, we can see at 24/06/22 3PM BST, a big move has occurred (highlighted in green showing upward move) leaving an inefficiency area that needs to be filled. The high end of the inefficiency area was reached in the following candle as there was no gap between that candle's open & the previous big candle's close. The low end of the inefficiency area was finally reached almost 4 hours later, at 6:55PM BST.