Periodization is a process of preparing for ultras by building specific skills at specific times. It usually begins with an endurance base-building phase, followed by a hill climbing and descending phase (or strength training), followed by a speed-building phase.
The idea works fairly well. By introducing each concept separately, you can develop each skill faster than if you were to do all three simultaneously. You begin with the most general skills (running long distances). As your goal race nears, you hone the specific skills needed for the ultra.
I used a system of periodization for my first ultra. I spent 12 weeks building my endurance base, 6 weeks working on speed, and 6 weeks working on specific trails. Note I didn't do any hill training. It was a huge mistake. The hills on the course absolutely killed me. Live and learn.