The idea is that what is normal is human consensus, that is, a social construct. According to Foucault, those in power decide what this normal is even though there is no valid reason for them to be in power more than for us to be. This domination and oppression mostly comes in the form of repression of sexuality and shows of power, and these things are invisible because they are considered to be normal, so people who don't agree with it are considered to not be normal which ostracizes them, even though they're just as normal as everyone.
