What is the UN migration pact about ?

Basically an agreement to say that borders don't exist and nations don't have sovereignty over themselves. Basically that you owe migrants the right to come in as they please and they own the country as much as the native born citizens.

Pretty fucked up

It's about cooperating to manage migration in a better way, rather than each country having a separate policy. The hope is that the share of migration would then also be split more fairly (rather than, say, Germany take many more migrants than other countries).

It's probably caused news because Donald Trump has pulled America out of the process and is trying to encourage others to stop it. Far right protesters (who seem to think anyone who moves from one country to another can only do so for the promotion of terrorism, despite many of them living in a different country than their ancestors) are also against it.

At its root, they are trying to eliminate all borders.

They're trying to route huge flocks of birds around areas of dense human population.

No clue