What does thickening of the colon lining indicate?

Depends: it could be nothing (just underdistention of the colon), or it could reresent inflammation of the colon, even cancer. it depends on the imaging technique used, the length and severity of thickening.

Thickening on x-ray: The differential includes: inflammation (colitis, diverticulitis), infiltrative disease (like cancer), muscular contraction (like spasm), more. Don't put off a colonoscopy for definitive evaluation. In the meantime, for your piece of mind, a home FIT (fecal immunochemical test) can be performed & will pick up 79% of colon cancers with a single screen (the rate is closer to 95% for colonoscopy).