It's like rips in space that connect two geometric areas. Imagine you're in a game and you're in a cave and there is a portal. Through it you can see a tropical forest, if you walk through that portal, you will be taken there instantaneously. And now imagine if those are dynamic as in they can be made anywhere and in any position. It's really weird to describe. It technically allows for deformable terrain since all geometries are handled dynamically, as opposed to BSP engine where everything is rigid in a BSP tree. The Red Faction engine was a portal engine. It's just really hard to work with them apparantly which is why the game was scrapped the first time. Luckily I was able to get my hands on the PC Gamer article from a lucky subscriber and the dynamic portals are definitely in.