CTR has different meaning depending on your ad unit
Pop / interstitial (DSP, myDSP)
It is the percentage of the available market you get through the bidding system. That’s why here, the CTR is your win rate, i.e. the percentage of pops you got vs the bids you made (click/impression).
For example if you get 5% of CTR, you have won 5% of all the bids you made.