PBR

Physically-Based Rendering

Albedo is pure colour, no lighting information baked in like we used to do with diffuse maps.
Roughness is calculated in a specifically measured way and requires a 0-1 input only. Gloss can interpolate that input to a greater range where Roughness "shouldn't".
Reflectance is again, a measured value for a material. there are plenty of sources online which you can get the measured reflective colours for various materials. typically speaking, most non-metals fall into a white 0.04 range, while metals (having no albedo and being pure reflection) have much higher, coloured reflectance. Specular is something that, along with gloss, artists have to fudge to get a pleasing look.
Overall, PBR is more about measured results rather than mixing in 3 different maps in different proportions to achieve a look.

Tutorials