A locus is a set of points that satisfies a specified condition. Here are some loci you need to know:
Theory
All points on a circle have the same distance from its center point.
The sum of the distances to the two focal points is the same for all points on the ellipse.
Equidistant Line:
All points that are equidistant from two specified points.
All points equidistant to a specified straight line.
Bisector:
All points equidistant to two specified lines.
Every angle $v$ that spans a line segment. Added together, these angles create a circle.
On exams, you may be asked what locus is being described. For that reason, it will be useful to remember what defines the different loci.