What it is
Every thread of silk a Bombyx mori silkworm spins is really two proteins in one: fibroin, the strong core filament, and sericin, the softer, water-soluble protein that coats and glues the fibroin strands together. Sericin can make up a fifth to a third of a cocoon's weight and gives raw silk its slightly stiff, gummy feel before processing.
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Before the silkworms turn into pupae, they are sacrificed, and the cocoons are recovered for silk extraction.
That step is standard commercial silk farming, not an unusual practice: reeling a cocoon into one unbroken filament only works if the pupa never chews its way out and breaks the thread, so cocoons are killed before the moth can emerge. Sericin is not a plant extract or a secretion collected without harm to the animal; it is a protein recovered from a farmed and killed silkworm.
How it ends up in cosmetics
Textile mills degum silk in hot water, sometimes with soap or enzymes, dissolving away the sericin coating to leave the smooth fibroin fibre that becomes fabric.
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Sericin, removed during the degumming of silk, is considered waste or by-product
Rather than send it down the drain, manufacturers increasingly purify and sell that leftover sericin for its moisture-binding, film-forming properties, mainly in moisturisers, serums, sheet masks and haircare. It usually appears simply as "sericin" or "silk protein" on an ingredients list, with nothing to suggest it began as cocoon waste from a silk mill.