Page out is when the OS takes a page of memory from the RAM and stores it on the hard drive. When the OS needs that memory back, it pages it back in. The paging process takes time because while RAM is very fast, comparatively, the hard drive is very very very slow. Ideally, everyone would have enough RAM so that the OS never had to page.
It usually does this when there isn't enough RAM, but per this thread, sometimes it does this when there is still availabe RAM.