What does 约谈 mean as a penalty?
约谈 is not a penalty, but it indicates or implies or leads to an subsequent penalty, like hold by authorities.

约谈 just means someone who are in a higher authority dated you to talk about some problems. In Chinese culture, it is not good, but not very serious. 

It may lead to a subsequent penalty, if the higher authority think your problem is serious or you are not honest on your problem. Sometimes 约谈 is just a warning, it depends on what you talked to them. You can't tell them all and you can't tell nothing, you can't tell them the truth and you can't tell them a total lie, they will all result in a very bad situation. So how to talk in a 约谈 is an art :) 

If you see an officer is 约谈 in a newspaper or official website then his political life is in danger, he may be charged by some of his misdoings soon. But if he is an entrepreneur then he is OK, he just need to adjust his entreprises to follow the government's will or cooperate with the government. Sometimes government authority 约谈 entreprises or entrepreneurs.