What does this mean?

"such and such is not responding due to a long-running script"?

I get the choice of clicking "stop script" or closing the pop-up bar. It doesn't matter which I choose, it comes right back, and I can do nothing with the page, including close it. I usually have to close all tabs. Sometimes a page will work for a few minutes and sometimes it won't even load. Or the original page works but the links on the page won't.

I've been getting this on almost every page I open for the last day or two. It's a miracle this one's working.