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?


It is the identity operator, $IX_t=X_t$, and is typically used in ARIMA type formulas where you also have the backshift operator $B$ (sometimes people use $\nabla$ for the backshift), or polynomials in $B$. Essentially, it's a way to make notation more compact.
For instance, if you have a time series $(X_t)$, then
$$(I-B)X_t = IX_t-BX_t = X_t-X_{t-1},$$
and
$$(I-B^2)X_t = IX_t-B(BX_t) = X_t-X_{t-2}.$$
