Focus-stealing is that incredibly annoying behavior of certain window managers in various operating systems. More specifically, it's when a newly mapped window yoinks keyboard focus to itself, without any regard for what previously had focus, or what the user is doing.
This drives me positively buggy! One minute, I'm happily typing in my password for some random website in Firefox, while waiting for this pokey old Pentium to wake up and start mIRC. Suddenly, though, mIRC wakes up, GRABS my keyboard focus, and before I know it, I've fed mIRC several random keystrokes and joined the wrong IRC net. It could be worse, though - Firefox doing this, while I'm trying to ssh back into my home box using PuTTY, could result in my password getting blabbed to the entire catbox!
It's not all the fault of the OS or WM, though. Some apps are absolutely heinous about this. Most little VBScript hacks are bad about this too. Under Unix, some window managers even have a feature to intercept and block some apps' demands for focus, if they detect that the user is typing (or has been in the last 2-3 seconds). This is a very nice feature. Microsoft, are you listening? Apple gets this mostly right. I'm not sure if Mac OS X has focus-stealing protection, or if it's just that the apps are well-behaved, but there's a lot less of this crap there. There's a lot fewer modal dialog boxes, too. (No one app should seize keyboard control completely. Trust me on this one.)