I am working on a problem that I think ARIMA modeling could be useful for, and am researching the theory behind ARIMA. I came across this website that says:
ARIMA(p,d,q) forecasting equation: ARIMA models are, in theory, the most general class of models for forecasting a time series which can be made to be “stationary” by differencing (if necessary).
My question is, what does this mean? When and why is it needed to make a time series ¨"stationary", and what is differencing?