CPI for publishers is the revenue or money you get per view of a web site.
For example, every page written on your site gets 500 views at a $10 CPM rate, then the CPI from the current page will be the following:
Cost Per Impression Rate ($) = CPM rate ($)/1000
If an advertiser sets $10 CPM rate then your earnings from per view will be = $10
CPM/1000 views = $0.01
Now, 500 views will earn you 500*0.01 ($) = $5