I would define it as speech that threatens or encourages violence against a group of people on the basis of their race, gender, sexual orientation or religion (e.g., blatant KKK-style rhetoric).
And, personally, I would go further and say that also includes both people who directly dog-whistle violence and people who intentionally provide a set of ultra-biased descriptive claims that lead the listener into making the violent normative claim themselves (e.g., Jordan Peterson, bell curve people, ethnostaters, etc).
I am definitely more on the extreme side of that, though. I don't value the American-style true freedom of speech; I don't think it leads to a more productive, happy society than those with minor restrictions like in Western Europe.
Most people who talk about hate speech aren't trying to thought police you, regardless of what angry dudes screaming about SJWs would have you believe - e.g., those "laws against misgendering" aren't saying you can't be an аsshole, but rather they are trying to move transgender people into a similar style of protected class that we give black people (i.e. you cannot intentionally misgender trans people in the workplace to purposefully create a hostile environment because they are trans, you can't harass or discriminate against trans people when it comes to being a landlord, etc).
Almost literally nobody is out there saying that you should arrest people who offend you. This is a total strawman. I hear people like you complaining about those people infinitely more than I actually hear those people do what you act like they do — and I even intentionally associate more with the kind of people that would stereotypically be your 'SJW' crowd.