It's very simple. Impressions are when someone looks at the page that list items when doing a search. If I click on the first 2 pages doing a search and your item is on one of those two pages it counts as one of your impressions.
Click through is when I see those items on the page and decide your price is reasonable and the picture looks like what I want. When I click on your item it counts as one of your click throughs.
If you get lots of impressions and no click throughs most likely buyers are not interested in your item. The main reason is either your price is too high, or your picture looks bad.
