What exactly do we mean by not verifing the transaction on optimistic rollups?

I have heard many times that transaction are not verified, it's assumed that the transaction is right. What exactly are we not processing/verifing in a transaction. We will calculate the state change, we will have to run the smart contract if any function is called. Or it's not like I can send eth without having it. What are exactly are we skipping, which results in so much increase in transaction through put?