Octocrylene is a chemical sunscreen ingredient that, once absorbed into the skin, captures harmful UV rays before they can cause damage to the underlying skin cells. Octocrylene mostly offers protection against the shorter UVB rays (those responsible for burning) and not as much against the longer UVA rays (those responsible for signs of aging).
Octocrylene is quite photo-stable and doesn't break down or change its molecular structure in the presence of UV light, as some other chemical sunscreen ingredients do. Octocrylene can also help improve the water-resistance of a sunscreen.