CTR: Click Through Ratio
Impressions: The amount of times it is displayed
So lets say you have a webpage. Then Google can show your page in its SERP's (using the snippet they have created). The amount of times it is shown to the Google visitors is called: Impressions.
When Google users click your snippet (from the search engine results) they "click through" to your website, thus: click though ratio, being the percentage of users that are presented with your snippet and that actually click it.