What does "Lag" mean in terms of Cross Correlation?
using the simplest definition of the cross-correlation:

$$ R_{xy}[k] \triangleq \sum_n x[n]\,y[n+k] $$

the "lag" is the displacement $k$.  (i am being deliberately vague about the limits to the summation.)  in the correlation $x[n]$ is lagging behind $y[n]$ by $k$ sample periods.