UV resistant filaments do exactly what it says on the tin, they are resistant to UV light. In other words, they will not degrade due to prolonged UV exposure. UV degradation can take many forms, including discolouration, loss of transparency, increased brittleness and even cracking.
In some cases, UV resistant plastics have been made by adding UV stabilisers which absorb the energy from the UV light and dissipate it gently, rather like sunscreen! Some plastics are inherently more UV stable, for example ASA is a UV stable chemically similar alternative to ABS.
UV resistant filaments are primarily used for products that will be exposed to lots of sunlight during outdoor usage, anything from a vehicle part to a garden gnome!