SCLK kernels correlate spacecraft clocks to solar system time. These files are necessary for things like interpreting CK Kernels, which encode observations using Spacecraft clock times.

More information can be found by downloading the LSK/SCLK tutorial:
https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/16_lsk_and_sclk.pdf