Surface geometry kernel is just the part of the code that handles its native geometry. ICEM CFD's geometry kernel supports two primary geometry types, faceted and bspline...
Faceted geometry is made up of lots of little triangles. This data can come from scanners or some low end geometry creation tools. These usually give a rough interpretation of the surface. Common types include, STL, VRML and various mesh types which can be used as geometry. Some CAD formats, such as DXF, can be faceted or bspline.
Higher end cad tools produce bspline
geometry which is much higher quality... Common sources would include low end options such as IGES/STEP or ACIS and high end tools like Pro/E, NX, Solidworks, etc. These are usually represented as large surfaces, each with their own uv space.
The mesh editing and some ICEM CFD meshers, such as the octree tetra mesher, do a behind the scenes conversion to represent bspline geometry as faceted for easier projection... The refinement of this behind the scenes conversion is controlled by the tri-tolerance. Look that up in the help if you need to know more, but the defaults are good for most users and you probably don't need to change it unless you don't feel the mesh is properly capturing the curvature of your model.