Basically:

• Republicans gerrymandered the state legislature so that they can’t be removed via an election
• Democrats elected a majority in the state Supreme Court which wants to rule that gerrymanders violate the state constitution
• Republicans are therefore planning to impeach one of the Democratic justices, but never bother with a trial, so that the justice can’t rule on cases, and no replacement can be appointed
• The US Supreme court has in the past ruled that what they want to impeach her for (expressing her opinion about controversial constitutional issues during an election) has to be allowed under the US constitution, but at least one of the current crop of Republican justices there has expressed the view that he doesn’t care, and would side with the Republican legislature