Validating in Commune means having a registered key with stake that is sending requests to miners to score their outputs on a offchain validation system to evaluate the relative value of each miner and calculate their weights.
The validation system is unconstrained in its complexity, computational cost and approach. However, its computational cost per evaluation has to be below the computational cost of miners per output to make sense.
With the exception of subnet 0, all validators run on Yuma Consensus making their Stake dividends based on how in-consensus their weights are. This enforces a uniform validation methodology across the subnet, as all validators have to reach approximately similar conclusions on the value of each miner.
If you are a subnet validator, for you validating means operating the validation system provided by the subnet founder and ensuring its functioning and quick updating to the latest changes.