what's the meaning for the extent changes for the an Density

from the msdn ,the  definition of Scan Density is "Best count is the ideal number ofextent changes if everything is contiguously linked. Actual count is the actual number ofextent changes. The number in scan density is 100 if everything is contiguous; if it is less than 100, some fragmentation exists. Scan density is a percentage." ,

So what's the meaning for the "extent changes" , how to explain it?

http://msdn.microsoft.com/en-us/library/aa258803(v=sql.80).aspx