For the rp2 pumps you want to make sure that the area you're pumping in is closed off from the bottom as it tends to fill from the bottom up. What this function does is if there's a cave opening connected to where you're pumping the lava it will fill up from the bottom of the cave. As for what he's saying about enclosing the lava lake, he probably means to make sure there's a roof on it when you start pumping so you don't accidentally pump lava outside of where you want to put it, since the lake can overflow if you're pumping from a large source, causing much havoc and mayhem.