Validators are independent physical persons or entities - not part of the AdEx Platform, who are responsible for tracking ad impressions/clicks and cryptographically signing confirmations of the impressions/clicks/earnings. Validators derive information about impressions/clicks via the AdEx AdView module, which they can later verify.
Once the transaction is verified by the majority of assigned validators and recorded on the payment channel, it cannot be changed, reverted, or manipulated. There is clear proof of the transaction which inevitably provides stability and guarantee for the advertising parties.
A validator stack is a collective term for all off-chain components responsible for handling events, managing OUTPACE channels, and generating analytical reports.