This depends a bit on the intensity and environment.
Welding arcs are bright enough to cause blistering in the eyes and to "sunburn" your skin; if you stared long enough you could blind yourself, but you'd have to be pretty ignorant to do so.
The better welding helmets are auto-darkening, and do this with a liquid crystal element similar to that used in a liquid crystal display like on some televisions/computer monitors. In either case, an electric field is used to change how transparent/opaque the crystal is, and thus how much light is absorbed vs transmitted. On a display, a driver controls this on a per-pixel basis to filter a backlight to create graphics. On the welding helmet, an optical sensor determines how intense the incoming light is and adjusts the opaqueness of the visor to compensate.
This isn't a perfect method, however. The energy of the light is still going somewhere, and with an opaque crystal layer the "somewhere" that it is is going is heat; strong enough light could damage the mechanism. Additionally, the electronics will have some limited operating temperature range, and may be susceptible to other forms of radiation. Astronaut helmets, for example, use a thin layer of gold to reflect the light, as opposed to absorbing it, which limits the amount of heat input and also doesn't fail if hit with (reasonable quantities of) other forms of radiation.
Even that kind of reflection wouldn't be enough, however, if you're talking about light from a high-powered laser, which would burn through that thin metallic layer. At those intensities, typically what is used to reflect light would be a dielectric mirror, which basically uses multiple layers of material that each reflect some amount of the light, and are stacked in thicknesses designed so that the wavelengths match up at each layer. This would be a bit difficult to use as protective covering, however, as the distance between layer boundaries changes with the angle at which the light comes in, and you'd also need to know the frequency in advance.