Oooh, I'm glad you asked.
It describes a 'window' in public opinion, which shifts around. Ideas range from unthinkable through acceptable to accepted/policy. Unthinkable ideas are unthinkable, and so not worth debating. I suppose this is like that feminism quote above - "first they dismiss us..."- so by the time they bother fighting us, we're closer to being Accepted Policy, right?
There is a concept of moving the window, such as deliberately promoting ideas even less acceptable than the previous "outer fringe" ideas, with the intention of making the current fringe ideas acceptable by comparison
So, like, we're all discussing social policy around rape prevention programs. Debate is about whether to change offender behaviour (feminist perspective) or potential-victim-behaviour (everyone else perspective), short skirts, minicabs, blah blah, that's the Overton window.
And then someone comes along and says "I think most of the time it's made up, women want sex and then change their mind" and AS SOON as people start putting their energies into disproving this, the Window has shifted. All of a sudden, the idea that you don't need rape prevention programs but rather rape-accuser-prevention programs becomes part of the window. And because the window is always about the same size, it means that an idea on the other side - usually, the idea that we should focus on the rapists - gets edged out. The new window is "should we police female behaviour to stop accusations, or police female behaviour to stop them being vulnerable".
Again, I'm reminded of that thread recently, where everyone was debating something sensible (no idea what, maybe it was the how has feminism improved your life thread) and daftpunk came along and said, literally, that men were demonstrably clever and superior to women. And people debated this, like it was worth debating! All of a sudden the Window included that idea.