Per 1K Imps. is the same thing as CPM or Cost Per Mille.
Let's use an example to understand better how this works for advertisers:

PsdTuts+ is selling ads with a cost Per 1K Imps (CPM). The CPM in our case is $1.25.
If you'd like to purchase, let's say 100,000 impressions, the total cost would be 100 x 1.25 = $125.
That's it!
More resources: http://en.wikipedia.org/wiki/CPM
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