I have hard time understanding the LINEAR part. Found something like this:
Linear property of OLS estimator means that OLS belongs to that class of estimators, which are linear in Y, the dependent variable. Note that OLS estimators are linear only with respect to the dependent variable and not necessarily with respect to the independent variables. The linear property of OLS estimators doesn’t depend only on assumption A1 but on all assumptions A1 to A5.
But it doesn't tell me much, I still don't understand. Can you try to explain in some simple intuitive way?