What are the i,j,k components of a 3d mesh on plot.ly (online)?
Most mesh drawing tools use triangles to draw meshes, for technical reasons.

You could simply specify the coordinates of the three vertices of each triangle, but that is a bit wasteful in terms of memory.

What is done in many places is to specify a list of points in space, with their coordinates, and then specify a list of triangles, using indices of the points in the first list. 

The exact same thing happens here. The x,y,z parameters form a list of points. The I,j,k parameters indicate triangles, using IDs in the list of points. 

Have a look at the function documentation for more details:
https://plot.ly/python/reference/#mesh3d