FK kernels describe relations between “frames”… Which are a little like Scenegraph nodes in UE or Unity.

These Frames relate the orientation and physics locations of varous objects relative to a reference. Individual Spacecraft will often have associated FK kernels to describe the orientations and positions of spacecraft parts relative to the spacecraft’s “Bus”.

More information can be found by downloading the SPICE FK Kernels Tutorial:
https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/21_fk.pdf

As well as the frames tutorial:
https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/17_frames_and_coordinate_systems.pdf

And using frames tutorial: https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/22_using_frames.pdf