Can someone explain what's radical about a 'terf'?

There is nothing radical about centring women in feminism.

It's only Liberal Feminists - or LibFems - who think it is, as far as I can see and it all went wrong around 3rd wave feminism, not that I'm an expert as I'm behind on my reading.

Check out

4thwavenow.com/

Radical doesn't describe the person.
Radical means that the problem is in the structure of society. As opposed to liberal feminism which is an "individualistic form of feminist theory, which focuses on women's ability to maintain their equality through their own actions and choices."

Copied from Wikipedia cos I don't know much else.

en.wikipedia.org/wiki/Radical_feminism

Radical feminism is a perspective within feminism that calls for a radical reordering of society in which male supremacy is eliminated in all social and economic contexts.

Radical feminists view society as fundamentally a patriarchy in which men dominate and oppress women. Radical feminists seek to abolish the patriarchy in order to "liberate everyone from an unjust society by challenging existing social norms and institutions."

Sorry, that should have been 2 main schools of feminism.

Radical means to the root, Latin explanation for the actual word. The root of a feminism is to centre women in your discourse.
The acronym TERF, is a useful example of the hatred that men feel towards women that oppose them.
Those four letters T.. E ..R..F.. are now used as an derogatory insult towards women that centre Women and don't believe that human beings can change sex.
In this strange world it seems that if you believe in Women's rights you are anti trans. Odd

There are 2 schools of feminism, radical and liberal. In this context 'radical' means 'root', as in 'lets get to the root of the problem'.

Liberal feminism is more about the choices made by the individual, and radical feminism says that women are discriminated against by the hierarchical system 'patriarchy'.

So as an example, a liberal feminist might say that 'sex work is work' and is empowering.
A radical feminist is more likely to say that sex work is a last resort kind of 'choice' made by women who lack other choices. If they had access to better resources such as a universal wage, childcare, or equal pay, women might choose other types of work.

Radical feminism is a type of feminism. There are many branches of feminism. One of the experts will be along to explain far better than I could!