TERF stands for Trans-Exclusionary Radical Feminist.
Yes, that is a lot of long words.
The word is used to describe women who don't believe it is straightforward, easy, or even possible, to change sex.
Now, the degree to which these people are right, and even if they are, I am not competent to judge. All I know is common sense and kindness.
But I do know that throwing insults at people is rarely a good way to win round anyone.
Sunday Rest: TERF. This word seems to have been popularised in 2008 by the trans-inclusive cisgender radical feminist (even more long words!) blogger Viv Smythe. To begin with TERF was a neutral and technical term, but it soon extended its meaning to cover any woman with doubts, worries, or concerns about the sex-change process. At the same time, the word became an insult.
It's now sometimes written terf.