These can be interpreted as desiccation cracks, filled with calcium- and/or magnesium sulfate (Ca-/Mg-SO4, seen as white material).
Methods used, for brevity citing from the below pop science link:
"... The team took a close physical and chemical look at those polygons using Curiosity's Mastcam, Mars Hand Lens Imager, ChemCam Laser Induced Breakdown Spectrometer (LIBS), and Alpha-Particle X-Ray Spectrometer (APXS) ..."
I believe this setting has led to the hypothesis of a once dried out lake on Mars.
https://sciencesources.eurekalert.org/pub_releases/2018-04/gsoa-cam041918.php
https://www.jpl.nasa.gov/news/mars-rover-curiosity-examines-possible-mud-cracks
