Microaggressions are everyday acts of violence and oppression that people encounter. They are specific encounters between people of different identities (race, class, gender, sex, orientation, culture, ability) and are non-physical, typically involving demeaning implications or subtle insults against a minority identity.
Frequently non-verbal, microaggressions are simply the small ways that folks with power intentionally or unintentionally make it clear that opposed identities are unwelcome, not on par or even not respected. Commonly this is "colourblindness" ("I don't see you as black, you're just a person") or denial of bias ("I'm not homophobic, I have a lot of gay friends").