CR stands for Conversion Rate. CR simply means the average Conversion Rate. The idea behind CR is to give you an idea on how the offer is performing compared to other offers on a specific affiliate network or program.
The Conversion Rate is not a guarantee of how the offer will perform if you run it.
Not all networks provide this useful metric on their platform. Sometimes though if you ask your account manager at the affiliate network they may provide this metric. On the network directory you find contact details for almost all the networks listed so you can easily get in touch with them to find out if they can provide you CR metrics for offers.