Basically a soil particle has three layers; the first layer is of air, 2nd water and 3rd is of solids.
Now to build a structure on a soil base there must be minimum air voids(to avoid settlement). We eliminate the air voids through compaction. But no matter how hard we try we can’t eliminate all the air voids and get zero air voids. But theoretically speaking we can calculate the dry density for zero air voids.
Thus draw a graph for dry unit weight and moisture content where the peak represents OMC(optimal moisture content) while there is also a straight line representing soil which is 100% saturated with zero air voids. This straight line is called a ZERO AIR VOID LINE.
Note that zero air void line cant be achieved in real life.