Transphobia is prejudice against trans* persons. Much like sexism, it is a problem in geek communities.
Why is transphobia a feminist issue?Edit
Aside from the fact that prejudice in general is a problem, there are reasons for feminists to be involved in opposing transphobia specifically.
One such reason is that transphobia usually expresses itself as trans-misogyny: the prejudice against trans women. As a type of prejudice faced almost exclusively by women, it is certainly a concern for feminists.