Not a doctor, so take with a grain or shaker full of salt.

Extremely simplified, it means you have some unusual observations found on the scan that need more expertise than he has, thus the referral to a GI doctor. From here, the GI doctor will likely do some sort of lighted visual examination of the area - possibly a colonoscopy, but more likely a sigmoidoscopy (a procedure to examine the insides of the rectum and the anus) or an anoscopy (a visual inspection of the anus) - depending of location and size of the unusual areas. In all these procedures, they may take a sample or several of the unusual tissue and have it analyzed for cancer (biopsy).

BTW - I'm sorry to bring up the c-word because the first reaction is fear.
It's too early to worry/panic about whether this is cancer. Other (non-cancerous) things can cause this, so be patient - wait for the GI doc and test results.

Hope this helps