It's just an alternative way of application switching. On Macs you have several ways of switching between windows. Most of the ways exist with variation on other systems also.
1) Clicking a window to bring it in focus and to the front (like on Windows and also like some Unix/Linux variants)
2) Clicking on an application's dock icon brings its window(s) to the front and also focuses them (analogous to clicking on a taskbar entry in Windows and again in some Linux variants)
3) You can use Apple-Tab to cycle through open applications and Apple-` to cycle through open windows of the current application (analogous to alt-tab and ctrl-tab, etc)
4) You can do it visually using Expose (loosely analogous to the visual system in Vista and also to the one that I think is part of compiz in some linux distros). So for this last one, the idea is that you manipulate the screen so that you can see all the windows that are open at once, and pick the one you want.
If you're fairly comfortable with the mouse, like a lot of us are, many of us like to set it up so that the mouse switches to Expose views by going to the corner of the screen (you can do this from System Preferences). Otherwise, it gets activated with function keys (which one is which is also listed in system preferences. It sounds like you're accidentally hitting the function keys.
To be honest, I avoided it for the first few months on my Mac and ultimately started using it predominantly, although I use all four methods interchangeably. After a while, it became very intuitive to me, although occasionally I do it inadvertently.