Carnosine

Though its name does not reveal it, Carnosine is a peptide, a small, two amino acid (β-Ala-His) one. It is naturally present in high concentrations in muscle and brain tissues, but the one used in cosmetic products is biomimetic, meaning that it is synthetically produced in a lab to copy the natural thing. 

A 2017 review paper on topical peptides writes about Carnosine that it is a "well-documented aqueous antioxidant with wound healing activity".  

Other than that we can write about manufacturer-done in-vitro (in the lab, not on real people) studies that show Carnosine to have anti-glycation properties. Glycation is the not-so-nice process that happens when we bombard our body with too much sugar that results in damaged body proteins and eventually in more wrinkles.  

Also, a manufacturer done in vitro study shows that carnosine might have collagen-boosing magic power. However, the 2017 research paper also mentions that even though Carnosine is a small molecule, it is water soluble and does not penetrate the skin past the top layers so we have some doubt if the collagen-boosting works in real life. We could find one anti-aging study made on real people that mentions Carnosine, but it was combined with a bunch of other anti-aging actives so it is pretty much impossible to know what Carnosine did or did not. 

One last thing to mention is that there is also a manufacturer done clinical study (done on real people) that shows carnosine being effective against the damages caused by infrared (IR) radiation. (source)