Is CTR a percentage of people that clicked on your link after seeing it from the impressions? 
E.g i have 152 impressions and a CTR of 0.66%. Does this mean 1.0032 people clicked on the link after itch.io showed them my title and thumbnail and stuff?

its not really important those are just numbers, it does not give any kind of advantage like youtube were you can get money by placing ads.

CTR is the rate an impression turned into a click. So your interpretation is correct. If you hover over the number/label a tooltip will appear explaining it.