it's basically gender dysphoria induced self hatred projected onto other people. some trans people will see some other trans person who exhibits traits they are dysphoric about, it will trigger their own dysphoria, and sometimes they will make it that other person's problem.

it is usually directed at people who are perceived to have the same gender as the person who's feeling it. often the person who triggers that secondhand dysphoria is purposely or incidentally gender nonconforming in some way, often non passing binary trans people and non-binary people.