Silica is a widely used cosmetic ingredient with the same chemical composition as sand found on beaches. One common function of silica is its ability to control viscosity. In certain chemical forms, this bulking agent can effectively thicken cream style products.
Another function of silica is its ability to act as an absorbent, which means that a lotion containing silica might have a lightweight, non-greasy application due to the silica.
As far as safety goes, silica is not a safety concern when found in skincare products. Some may be concerned about the particle size of some silicas and being able to pass through the skin barrier.
However, the particles used in cosmetics are too large to pass through.