The most basic observation to be made about spatial data is that it typically exhibits spatial structure. In statistical terms, this translates to the observation that spatial data are not random. Knowing something about a phenomenon at some location A, often tells us a great deal about the same phenomenon at a location not far away from A. Another way of putting this is that spatial data sets are correlated with themselves over distance.
When two variables x and y are correlated, then given the value of x for a particular case, I can make a good estimate of the likely value of y for that case. Similarly, given information about the value of some attribute measured at spatial location A, then I can often make a reasonable estimate of the value of the same attribute at a nearby location to A. This is due to spatial autocorrelation (spatial self-correlation). What is "nearby" is something defined by the statistician and defined mathematicaly by a geographical weighting scheme. Whatever the measure we use, we often refer to this statistical description of nearby observations as a spatial lag.
Much of the material we study in this course makes use of spatial autocorrelation in data, whether it is assumed or measured. Perhaps the best example is interpolation (see the upcoming Lesson 6), where we use the information only from nearby control points to inform our calculation of an estimated value at a location where no observation has been recorded. We do this because we expect nearby data to be more relevant than distant data. In kriging, this is taken one step further when one method of measuring spatial autocorrelation--the semivariogram--is used to improve further the estimates produced by an interpolation method.