A locus is just a group of points satisfying a rule of some sort. Simple loci are often shapes or lines.
e.g. the locus of points 1 unit distance from (0,0) is the unit circle.
e.g. the locus of points that are equal distance from (-1,0) and (1,0) is the y axis (a straight line)
I can't see the question you attached so can't git more help I'm afraid.