“Periodization” just means how you change your training over time instead of doing the exact same thing forever. Two common ways to do it: linear and undulating.
Linear
You move through phases in order. Start with higher reps and lighter weight for a few weeks, then move to medium, then finish with low reps and heavy weight. Each phase builds on the last, usually ending with a peak strength phase.
Good for: beginners who want something simple and predictable, or anyone building toward a specific event, like a powerlifting meet.
Undulating
You change reps and weight much more often — sometimes every session. Monday might be heavy (4-6 reps), Wednesday medium (8-10), Friday light and high-rep (12-15), and you keep cycling that.
Good for: more experienced lifters who want variety, or anyone training the same muscle multiple times a week with different rep ranges each time.
What the Research Says
A big review compared the two directly for muscle growth. Result: no real difference. Both build muscle about the same when total work is similar. Some older research hints undulating might be a tiny bit better for strength in experienced lifters, but the difference is small and the evidence is mixed.
What To Actually Do
Neither one is “better.” Linear is simpler to plan and track, and works well if you’re building toward a specific goal or event. Undulating gives more variety and suits people training a muscle group more than once a week.
Bottom Line
Pick whichever one you’ll actually stick to. Both build muscle and strength fine — sticking with it matters way more than which model you pick.