Residuals are the random fluctuations left over after trend and seasonality are removed.
Unpredictable, random, “residual” fluctuations
Due to random variations of Nature, Accidents or unusual events
Noise in the time series
They are what is left over after trend and seasonality are removed from the original time series.
You should not see a trend or seasonal pattern in the residual.
They represent short-term fluctuations.
They are either random or a portion of the trend or seasonality components was missed in the decomposition