Microaggressions are defined as the everyday, subtle, intentional — and oftentimes unintentional — interactions or behaviors that communicate some sort of bias toward historically marginalized groups.
The difference between microaggressions and overt discrimination or macroaggressions, is that people who commit microagressions might not even be aware of them.
Someone commenting on how well an Asian American speaks English, which presumes the Asian American was not born here, is one example of a microaggression. Presuming that a black person is dangerous or violent is another example. A common experience that black men talk about is being followed around in stores or getting on an elevator and having people move away and grab their purses or their wallets.
Oftentimes, people don't even realize that they're doing those sorts of things. And in fact, if you were to stop them and say, 'Why did you just move?' They would deny it because they don't recognize that their behaviors communicate their racial biases.