What does the I operator stand for in the context of time series modeling? I am reading a paper on parameter estimation for moving-averages, but I'm not exactly sure what the I operator underlined in red stands for. There is no explanation for it anywhere in the paper. Is it the identity matrix?


The main answer by Stephan is correct, but it is odd to use the identity operator (with the symbol $I$) in a scalar context instead of just using the number one.  It would be simpler here if they just said that:
$$(1-B)X_t = X_t-X_{t-1}.$$
