They are subtle: There is a wide range of acts that can insult, exclude, and harm. Many of them are obvious, but many are in fact quite subtle, hard to identify, and challenging to speak about.
They are acts: They are things that people say and do and we can talk about why those particular things might have been a problem. We can get better not doing those things.
They serve to exclude: Subtle acts create exclusion rather than inclusion. This is a problem on an individual level, as well for the larger culture where the repetition of these subtle acts reinforces and maintains systems of power, inequality, bias and what is considered normal.