Periodization is the method where you change your workout variables during different periods or phases throughout the year.
You dont keep doing the exact same routine over and over forever.
Undulating periodization is a type of periodization where you change your rep ranges (and the corresponding weight you must use - heavy, medium or light.
Undulating periodization is basically a fancy schmancy scientific term for using a heavier - lighter, or heavier - medium, or heavy-medium-lighter training system.
Its different from block periodization in the way you change reps / weights inside every week.
In block periodization you might do a long phase of all moderate or lighter training with more reps.
Like 4 weeks at least, or maybe months. Then switch to a long block of heavier training.
With undulating periodization every other workout you rotate heavier and lighter workouts.
A quick example for a 2 day split/ 4 days per week:
Monday: upper body: moderate 8 to 12 reps.
Tuesday: lower body: moderate 8 to 12 reps
Thursday: upper body: heavy 4 to 6 reps
Friday: lower body: heavy 4 to 6 reps
The term comes from "undulate" as in, like a wave, up and down and up and down in the weights and reps.