So, I am a white dude who was pretty skeptical about the whole microaggression thing. But I changed my mind.
My girlfriend is biracial and just watching how strangers interact with her taught me a lot about the state of race relations in this country.
Microaggressions (I am going to abbreviate this as MA) are more like micro-transgressions. In the vast majority of cases, the people who commit MA are not seeking to hurt the recipient of the MA, they are likely trying to be pleasant and friendly. But there is something about the interaction where the person expresses a subconscious bias that the target is not normal.
A great example, is the question "What are you?" How often are you asked this question? Me, I've never been asked this outside of Halloween. My GF gets this question a lot, at the grocery store, at the dog park, etc. Why? What does it mean? Why is it hurtful?
The questioner never means anything bad by the question. It's honest curiosity.
Based on the conversation that follows (a nice chat about our respective ancestries) and the fact that I never get asked this question, they are really asking, "are you black, Pacific islander, asian, or what?" but have learned that that is a rude question.
Why is this an unpleasant experience? It is "othering". Yeah, what does that mean? Well, it means that it is inherently exclusionary. It means that the asker of the question sees you as fundamentally different. And not in a good "let's celebrate our differences"-way, but in a hidden-deep-in-our-limited-monkey-brain "is this person safe" kind of way. It means "are you going to mug me" and "do I have to worry about saying something racist that my white friends would give me a pass on". It means "Are you one of us?" with the presumed answer of "no". For most of us, all of this is subconscious.
People who get asked "what are you?" regularly know what the implicit questions are, even if the person asking them doesn't. They questions feel aggressive, but they aren't outright, in-your-face, aggression.
Ask yourself, honestly, when you meet a white person who speaks with a local accent, how soon do you ask about their heritage?