a) DHS / Customs & Border Protection want to unmask the person(s) behind the @ALT_USCIS account
b) They want to use an unnamed "investigatory tool" to do this
c) Twitter does not want them to do this, on (extensive) 1st amendment grounds
d) Twitter says the Supreme Court is on their side on this issue
The lawsuit is to stop DHS / CBP from using the tool.
The "tool" is a legal "tool" - an ability granted to the agencies to legally compel Twitter to do stuff.
The reasonable editorial is then that the Government wants to do this because they don't like being criticized or contradicted by these ALT_* accounts, and Twitter wants the accounts to stay anonymous because the criticism is good (both in terms of quality and effect), and taking away that anonymity is all kinds of bad.
(I'm totally on Twitter's side on this)