what does anti-black mean?

The personal, cultural, social, legal, and structural attacks on black people. A term that some feel should replace racism as it is more focused on black people alone rather than all people of color.

who can be anti-black?

Anyone and everyone. Even black people. Internalized anti-blackness is a real thing that happens within black people. It has a lot to do with the context they grew up in and the media's portrayal of black people over the years.

"I'm not racist. I support things like BlackLivesMatter and Anti-Police Brutality Efforts!"

Also take note of possible anti-black companies you give money to and media you consume. Because all of these systems are rooted in white supremacy. Fortunately, there are resources, but you have to do the work to stay informed as an active member of our society.