Click Through Rate means how many views it takes to generate one click. If a plug has a CTR of 1:500, it means that it takes 500 impressions/views from visitors, for one unique click. The lower the second number, the better the CTR is. So a plug with a CTR of 1:100 is ten times more clickable than a plug with a CTR of 1:1000.
The easiest way to improve CTR, is to upload a HQ clear thumbnail of your video/gallery/image/etc. You may also improve it by writing a better title and description. But remember that an image says more than a thousand words.