I see this word re at peak performance appear on this video (At 1:29) I didn't find any information about this expression, can you please help me to understand it?

Free radicals can speed up aging and cause health disorders, and chronic organ dysfunctions. With the boosted antioxidants, your production of white blood cells will also be improved. White blood cells are your protection against elements that attack your body. When they re at peak performance. your body's immune system will improved.

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When they're at peak performance

The apostrophe was missing.

they are

Clive