What does this mean?

That's either a typo or a misreading on your part.

It should be 480 lbs.

lbs = pounds.

So a chronic dieter can lose and regain 480 pounds by the time she's 35.

The abbreviation "lb" for a unit of weight is derived from the Latin word libra.
The Latin words libra pondo describe a Roman unit of weight similar to a pound.

lbs is the plural of lb

However one interprets it, it's an outrageous and ridiculous claim.

I dunno, it could happen.

Say she starts the chronic dieting at 15. Over twenty years, she gains 12 pounds every six months and then loses it again. That's 24 pounds lost/gained a year, x 20 = 480.

(Or make it that she gains 24 pounds every six months and then loses it again, depending on how you read the sentence.)

So, January 1st she's 182.
June 1st she's 170.
January 1st she's 182.

Etc.