Portals allowed humans to travel to and back form locations that they had been to before. They were often used to travel to other planets. The technology was obtained when humanity managed to take down there first Dalki ship and discovered a portal inside.
Before Del was to inform the students about the Portal training they would be participating in. He needed to explain to the students how the portals worked. After all, it would be a first for nearly every student there apart from a few of them.