I had a renal scan done and it said that I have mild cortical thinning of the kidney what does this mean

Cortical refers to the outer or cortex layer of the kidney. The following example should clarify: The Kidney

Your description means mild (non-specific) thinning in some part of the area shown in red (denoted "F").

Are there other terms contained within your information? i.e. diffuse, bi-lateral, focal, etc?

Findings :
The kidneys are symmetric in size and demonstrate symmetric contrast
Excretion. There is no radiopaque nephrolithiasis. There is mild fullness
Of the left intrarenal collecting system, which appears stable compared to
Prior examination. There are areas of mild left renal cortical thinning,
Particularly along the lateral cortex. There is mild contour irregularity
Along the left upper pole cortex which appears stable compared to the prior
Examination. A subcentimeter wedge-shaped cortical defect in the posterior
Left mid to lower pole cortex is also stable compared to the prior exam. On
The delay images, the ureters are intermittently visualized. There is focal
Short segment narrowing of the distal right ureter, just prior to its
Crossing over the iliac vessels (image 52, series 6) without any evidence
Of proximal or distal dilatation or internal filling defect. No surrounding
Mass is visualized. This is most likely due to peristalsis