some gesture im not sure about?

Hello. I am running Ubuntu on a MacBook Pro and it seems to be working pretty well, but I've got a few problems.

One of them.. somehow when I have my fingers on the trackpad I seem to be triggering some sort of gesture I am not aware of, which opens up the little window to switch between applications. I don't know exactly what gesture it is that I'm doing or how to turn off this setting, but it's very annoying. It will sometimes hide the current window that's open or switch to a different application. It seems to happen when I have two fingers on the trackpad but it doesn't always happen and I can't figure out what, exactly, is the gesture that I'm apparently doing.

Can anyone help?

Thanks!