Collagen, and what a cream can do with it
Collagen is on a lot of packaging and in a lot of headlines, and the gap between what people think it means on a label and what it does there is wide enough to be worth explaining.
What it is
A structural protein, and a large part of what makes young skin springy. There is less of it as time passes, which is a real part of why skin comes to look less full.
Why a cream cannot simply top it up
Collagen is a big molecule and the outer layer of skin is very good at keeping big molecules out. That barrier is the point of it. So collagen applied to the surface does not travel down and join the collagen already there, however sensible that sounds.
What it is doing in the bottle then
Sitting on the surface and holding water, which is a real and useful thing to do, and the same thing glycerin and hyaluronic acid do. Skin holding water looks fuller. That is worth having, it is just not the same as rebuilding anything.
How to read collagen on a label
As a humectant, and as a well chosen word. If the page around it promises to restore or rebuild your skin, the ingredient is being used to make a claim it does not support. If the page says skin looks plumper while you are using it, that is the honest version and it is achievable.