Credits and impressions are synonyms. 

A credit is used when an email is opened and the timer is displayed one time. For example, you send out 100 emails, and all 100 get opened 1 time. That would be 100 credits.

Typically we cache from the same device and IP address on our CDN. We use a CDN to cache the images for two minutes so each hit on the banner that is cached is not counted. 

The first open will count, the subsequent may be cached. For example, you receive an email on your machine and open it once you'll get one impression. Opening the same email again within 2 minutes is not going to trigger anything because it will be cached on the device. If you go back an hour later, it will count as an impression.