What is cluster centroid?

Hi, good to see you again, I'm BK , student of Handong Global University, South Korea.

I am reading and trying to mimic a paper "I-TASSER: a unified platform for automated protein structure and function prediction".

Unfortunately, I have some difficulties on understanding cluster centroid.

what is cluster centroid and how can we get it? and why it is important?

and I tried to use SPICKER then it gave me a result as below,
the number of cluster is 1.
total number of structure is 19992.
is cluster centroid a mean value of x, y, z axis of the cluster?
(is cluster centroid placed on that point; average point/area of x,y,z?)