What is a locus anyway?
Answer:
A locus is the set of points that form a geometric figure or a graph. This word is confusing due to its overly abstract nature.
Translation into 'english'
A locus is just a bunch of points that satisfy a certain condition or rule.
(Still pretty abstract, I know, but look at the examples below to try to better undersand what a locus is)
Examples of commonly studied loci
Shape  Rule/Condition Satisfied 
Circle  set of points that are the same distance(known as the radius) from the center of the circle 
Parabola 
a parabola is the set of all points that are the same distance from a point (called the focus) and a line(called the directrix). As shown in the image below:
