If you google or use Wikipedia to find the difference between Slack and Pants, you are most likely to either get into a labyrinth or facing a tedious rocket-science paper.
And today, we're going to give a dummy-proof explanation. And just forget about those difference in British English, American English or even Ancient Saxon. We'll be only talking in the modern, American English today:
In a narrow sense, Slack means loose fitted pants, with an implication of informal wear.
In a broad sense, Slack means exactly same as pants.
That's it. Nothing ambiguous. And feel free to use "Slack" in the same way of "Pants" from now on.