It basically means a trans person who cares a lot about appealing to conservatives/transphobes as "one of the good ones," in ways that are often damaging to other trans people.

Someone who is called a pick me might say stuff like "you don't have to use preferred pronouns if they don't pass" or "you're not a [man/woman] if you don't have [sex organ]." They might be against transition even though they have transitioned and don't regret it. They might excessively shit talk other trans people. They might "debate" a lot of conservatives and end up agreeing with them on various points.