What Gaventa is trying to say about power is that it is much more than what we usually think it is. When we think about power, or when we look for evidence that someone has power, we usually look only at what Gaventa would call the first face of power. We look only at issues that came up for decision, at which point we ask who won and who lost on that issue.
But Gaventa says that power goes well beyond that. He says that there are ways for power to be exercised behind the scenes, as it were. These are faces of power that we do not really see, but which are very important. The second face of power is one in which those with power can set up the rules so that decisions will never be made. For example, if a union wanted to organize a particular work force, the management might manipulate things so that there was never a vote taken about whether to unionize or not. This would not show up if we were looking for the first face of power. No election means no decision and no way to look at who won or who lost. But really, the management won by preventing the decision from happening.
A third face of power is even more subtle and difficult to see. This is the level where the powerful are said to be able to use symbols and ideologies to make sure that inequities are never even felt. The example here might be (to some) our current system. There is a big gulf between rich and poor in the US, yet the poor seem not to mind this for the most part. Gaventa would argue that this is because of the third face of power. Those in power have manipulated our national discourse so that we simply accept that this inequality is natural. We never come to decide whether we should accept it or not because we never even see it as a problem.
Overall, then Gaventa is saying that power isn't just winning votes or official decisions. Instead, it can be wielded behind the scenes to make sure some things never come to a vote or are not even perceived as problems.