The higher a circle is on the Y axis, the more machines in your LAN have particular files. The further a circle is on the X axis, the more ESET users have the file (ie. the more popular it is worldwide). The bigger a circle is, the more such files you have.
To illustrate it on a concrete example:
The red-marked circle means that you have quite many files that exist only on 1 computer in your LAN but are quite popular among ESET users since the LG popularity is 7 (1-10 mil. of users):