When light encounters a surface, a certain amount of it is absorbed, some may pass through the material, and the rest reflects. All of those functions are variable with wavelength- a material that is opaque to visible light may be transparent to other forms of radiation, like radio waves. This is actually where our perception of color comes from- it is the wavelengths of light that weren’t absorbed by and didn’t pass through the surface we’re looking at. (Sorry, I’m a nerd about this sort of stuff)

A laser is extremely concentrated light. Most materials can only absorb a certain amount of that energy. As such, a lot more of it reflects back off the opaque surfaces it encounters, even if they aren’t mirrors. For a strong enough laser, even that weakend reflection can do damage.